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_example/src/main.rs

@@ -1,12 +1,12 @@
 use nu_plugin::{serve_plugin, MsgPackSerializer};
-use nu_plugin_example::Example;
+use nu_plugin_example::ExamplePlugin;
 
 fn main() {
     // When defining your plugin, you can select the Serializer that could be
     // used to encode and decode the messages. The available options are
     // MsgPackSerializer and JsonSerializer. Both are defined in the serializer
     // folder in nu-plugin.
-    serve_plugin(&Example {}, MsgPackSerializer {})
+    serve_plugin(&ExamplePlugin {}, MsgPackSerializer {})
 
     // Note
     // When creating plugins in other languages one needs to consider how a plugin