Make plugin commands support examples. (#7984)

# Description

As title, we can't provide examples for plugin commands, this pr would
make it possible


# User-Facing Changes

Take plugin `nu-example-1` as example:
```
❯ nu-example-1 -h
PluginSignature test 1 for plugin. Returns Value::Nothing

Usage:
  > nu-example-1 {flags} <a> <b> (opt) ...(rest)

Flags:
  -h, --help - Display the help message for this command
  -f, --flag - a flag for the signature
  -n, --named <String> - named string

Parameters:
  a <int>: required integer value
  b <string>: required string value
  (optional) opt <int>: Optional number
  ...rest <string>: rest value string

Examples:
  running example with an int value and string value
  > nu-example-1 3 bb
```

The examples session is newly added.

## Basic idea behind these changes
when nushell query plugin signatures, plugin just returns it's signature
without any examples, so nushell have no idea about the examples of
plugin commands.
To adding the feature, we just making plugin returns it's signature with
examples.

Before:
```
        1. get signature
         ----------------> 
Nushell ------------------  Plugin
        <-----------------
        2. returns Vec<Signature>
```

After:
```
        1. get signature
        ----------------> 
Nushell ------------------  Plugin
        <-----------------
        2. returns Vec<PluginSignature>
```
        
When writing plugin signature to $nu.plugin-path:
Serialize `<PluginSignature>` rather than `<Signature>`, which would
enable us to serialize examples to `$nu.plugin-path`

## Shortcoming
It's a breaking changes because `Plugin::signature` is changed, and it
requires plugin authors to change their code for new signatures.

Fortunally it should be easy to change, for rust based plugin, we just
need to make a global replace from word `Signature` to word
`PluginSignature` in their plugin project.

Our content of plugin-path is really large, if one plugin have many
examples, it'd results to larger body of $nu.plugin-path, which is not
really scale. A solution would be save register information in other
binary formats rather than `json`. But I think it'd be another story.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu_plugin_example/src/main.rs

@@ -13,10 +13,10 @@ fn main() {
     // is added and used in nushell.
     // The steps are:
     // - The plugin is register. In this stage nushell calls the binary file of
-    //      the plugin sending information using the encoded PluginCall::Signature object.
+    //      the plugin sending information using the encoded PluginCall::PluginSignature object.
     //      Use this encoded data in your plugin to design the logic that will return
     //      the encoded signatures.
-    //      Nushell is expecting and encoded PluginResponse::Signature with all the
+    //      Nushell is expecting and encoded PluginResponse::PluginSignature with all the
     //      plugin signatures
     // - When calling the plugin, nushell sends to the binary file the encoded
     //      PluginCall::CallInfo which has all the call information, such as the

crates/nu_plugin_example/src/nu/mod.rs

@@ -1,15 +1,29 @@
 use crate::Example;
 use nu_plugin::{EvaluatedCall, LabeledError, Plugin};
-use nu_protocol::{Category, Signature, SyntaxShape, Value};
+use nu_protocol::{Category, PluginExample, PluginSignature, SyntaxShape, Value};
 
 impl Plugin for Example {
-    fn signature(&self) -> Vec<Signature> {
+    fn signature(&self) -> Vec<PluginSignature> {
         // It is possible to declare multiple signature in a plugin
         // Each signature will be converted to a command declaration once the
         // plugin is registered to nushell
         vec![
-            Signature::build("nu-example-1")
-                .usage("Signature test 1 for plugin. Returns Value::Nothing")
+            PluginSignature::build("nu-example-1")
+                .usage("PluginSignature test 1 for plugin. Returns Value::Nothing")
+                .required("a", SyntaxShape::Int, "required integer value")
+                .required("b", SyntaxShape::String, "required string value")
+                .switch("flag", "a flag for the signature", Some('f'))
+                .optional("opt", SyntaxShape::Int, "Optional number")
+                .named("named", SyntaxShape::String, "named string", Some('n'))
+                .rest("rest", SyntaxShape::String, "rest value string")
+                .plugin_examples(vec![PluginExample {
+                    example: "nu-example-1 3 bb".into(),
+                    description: "running example with an int value and string value".into(),
+                    result: None,
+                }])
+                .category(Category::Experimental),
+            PluginSignature::build("nu-example-2")
+                .usage("PluginSignature test 2 for plugin. Returns list of records")
                 .required("a", SyntaxShape::Int, "required integer value")
                 .required("b", SyntaxShape::String, "required string value")
                 .switch("flag", "a flag for the signature", Some('f'))
@@ -17,17 +31,8 @@ impl Plugin for Example {
                 .named("named", SyntaxShape::String, "named string", Some('n'))
                 .rest("rest", SyntaxShape::String, "rest value string")
                 .category(Category::Experimental),
-            Signature::build("nu-example-2")
-                .usage("Signature test 2 for plugin. Returns list of records")
-                .required("a", SyntaxShape::Int, "required integer value")
-                .required("b", SyntaxShape::String, "required string value")
-                .switch("flag", "a flag for the signature", Some('f'))
-                .optional("opt", SyntaxShape::Int, "Optional number")
-                .named("named", SyntaxShape::String, "named string", Some('n'))
-                .rest("rest", SyntaxShape::String, "rest value string")
-                .category(Category::Experimental),
-            Signature::build("nu-example-3")
-                .usage("Signature test 3 for plugin. Returns labeled error")
+            PluginSignature::build("nu-example-3")
+                .usage("PluginSignature test 3 for plugin. Returns labeled error")
                 .required("a", SyntaxShape::Int, "required integer value")
                 .required("b", SyntaxShape::String, "required string value")
                 .switch("flag", "a flag for the signature", Some('f'))