Change PluginCommand API to be more like Command (#12279)

# Description

This is something that was discussed in the core team meeting last
Wednesday. @ayax79 is building `nu-plugin-polars` with all of the
dataframe commands into a plugin, and there are a lot of them, so it
would help to make the API more similar. At the same time, I think the
`Command` API is just better anyway. I don't think the difference is
justified, and the types for core commands have the benefit of requiring
less `.into()` because they often don't own their data

- Broke `signature()` up into `name()`, `usage()`, `extra_usage()`,
`search_terms()`, `examples()`
- `signature()` returns `nu_protocol::Signature`
- `examples()` returns `Vec<nu_protocol::Example>`
- `PluginSignature` and `PluginExample` no longer need to be used by
plugin developers

# User-Facing Changes
Breaking API for plugins yet again 😄

crates/nu-plugin-test-support/tests/custom_value/mod.rs

@@ -3,8 +3,7 @@ use std::cmp::Ordering;
 use nu_plugin::{EngineInterface, EvaluatedCall, Plugin, SimplePluginCommand};
 use nu_plugin_test_support::PluginTest;
 use nu_protocol::{
-    CustomValue, LabeledError, PipelineData, PluginExample, PluginSignature, ShellError, Span,
-    Type, Value,
+    CustomValue, Example, LabeledError, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 use serde::{Deserialize, Serialize};
@@ -58,14 +57,24 @@ impl Plugin for CustomU32Plugin {
 impl SimplePluginCommand for IntoU32 {
     type Plugin = CustomU32Plugin;
 
-    fn signature(&self) -> PluginSignature {
-        PluginSignature::build("into u32")
-            .input_output_type(Type::Int, Type::Custom("CustomU32".into()))
-            .plugin_examples(vec![PluginExample {
-                example: "340 | into u32".into(),
-                description: "Make a u32".into(),
-                result: Some(CustomU32(340).into_value(Span::test_data())),
-            }])
+    fn name(&self) -> &str {
+        "into u32"
+    }
+
+    fn usage(&self) -> &str {
+        "Convert a number to a 32-bit unsigned integer"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name()).input_output_type(Type::Int, Type::Custom("CustomU32".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            example: "340 | into u32",
+            description: "Make a u32",
+            result: Some(CustomU32(340).into_value(Span::test_data())),
+        }]
     }
 
     fn run(
@@ -87,9 +96,16 @@ impl SimplePluginCommand for IntoU32 {
 impl SimplePluginCommand for IntoIntFromU32 {
     type Plugin = CustomU32Plugin;
 
-    fn signature(&self) -> PluginSignature {
-        PluginSignature::build("into int from u32")
-            .input_output_type(Type::Custom("CustomU32".into()), Type::Int)
+    fn name(&self) -> &str {
+        "into int from u32"
+    }
+
+    fn usage(&self) -> &str {
+        "Turn a u32 back into a number"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name()).input_output_type(Type::Custom("CustomU32".into()), Type::Int)
     }
 
     fn run(