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_query/src/nu/mod.rs

@@ -1,25 +1,25 @@
 use crate::Query;
 use nu_plugin::{EvaluatedCall, LabeledError, Plugin};
-use nu_protocol::{Category, Signature, Spanned, SyntaxShape, Value};
+use nu_protocol::{Category, PluginSignature, Spanned, SyntaxShape, Value};
 
 impl Plugin for Query {
-    fn signature(&self) -> Vec<Signature> {
+    fn signature(&self) -> Vec<PluginSignature> {
         vec![
-            Signature::build("query")
+            PluginSignature::build("query")
             .usage("Show all the query commands")
             .category(Category::Filters),
 
-            Signature::build("query json")
+            PluginSignature::build("query json")
             .usage("execute json query on json file (open --raw <file> | query json 'query string')")
             .required("query", SyntaxShape::String, "json query")
             .category(Category::Filters),
 
-            Signature::build("query xml")
+            PluginSignature::build("query xml")
             .usage("execute xpath query on xml")
             .required("query", SyntaxShape::String, "xpath query")
             .category(Category::Filters),
 
-            Signature::build("query web")
+            PluginSignature::build("query web")
             .usage("execute selector query on html/web")
             .named("query", SyntaxShape::String, "selector query", Some('q'))
             .switch("as-html", "return the query output as html", Some('m'))

crates/nu_plugin_query/src/query.rs

@@ -3,7 +3,7 @@ use crate::query_web::parse_selector_params;
 use crate::query_xml::execute_xpath_query;
 use nu_engine::documentation::get_flags_section;
 use nu_plugin::{EvaluatedCall, LabeledError, Plugin};
-use nu_protocol::{Signature, Spanned, Value};
+use nu_protocol::{PluginSignature, Spanned, Value};
 use std::fmt::Write;
 
 #[derive(Default)]
@@ -58,19 +58,19 @@ impl Query {
     }
 }
 
-pub fn get_brief_subcommand_help(sigs: &[Signature]) -> String {
+pub fn get_brief_subcommand_help(sigs: &[PluginSignature]) -> String {
     let mut help = String::new();
-    let _ = write!(help, "{}\n\n", sigs[0].usage);
-    let _ = write!(help, "Usage:\n  > {}\n\n", sigs[0].name);
+    let _ = write!(help, "{}\n\n", sigs[0].sig.usage);
+    let _ = write!(help, "Usage:\n  > {}\n\n", sigs[0].sig.name);
     help.push_str("Subcommands:\n");
 
     for x in sigs.iter().enumerate() {
         if x.0 == 0 {
             continue;
         }
-        let _ = writeln!(help, "  {} - {}", x.1.name, x.1.usage);
+        let _ = writeln!(help, "  {} - {}", x.1.sig.name, x.1.sig.usage);
     }
 
-    help.push_str(&get_flags_section(&sigs[0]));
+    help.push_str(&get_flags_section(&sigs[0].sig));
     help
 }