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/src/lib.rs

@@ -17,8 +17,8 @@
 //!
 //! ```rust,no_run
 //! use nu_plugin::{EvaluatedCall, MsgPackSerializer, serve_plugin};
-//! use nu_plugin::{Plugin, PluginCommand, SimplePluginCommand, EngineInterface};
-//! use nu_protocol::{PluginSignature, LabeledError, Value};
+//! use nu_plugin::{EngineInterface, Plugin, PluginCommand, SimplePluginCommand};
+//! use nu_protocol::{LabeledError, Signature, Value};
 //!
 //! struct MyPlugin;
 //! struct MyCommand;
@@ -32,7 +32,15 @@
 //! impl SimplePluginCommand for MyCommand {
 //!     type Plugin = MyPlugin;
 //!
-//!     fn signature(&self) -> PluginSignature {
+//!     fn name(&self) -> &str {
+//!         "my-command"
+//!     }
+//!
+//!     fn usage(&self) -> &str {
+//!         todo!();
+//!     }
+//!
+//!     fn signature(&self) -> Signature {
 //!         todo!();
 //!     }
 //!
@@ -71,9 +79,10 @@ pub use serializers::{json::JsonSerializer, msgpack::MsgPackSerializer};
 // Used by other nu crates.
 #[doc(hidden)]
 pub use plugin::{
-    get_signature, serve_plugin_io, EngineInterfaceManager, GetPlugin, Interface, InterfaceManager,
-    PersistentPlugin, PluginDeclaration, PluginExecutionCommandContext, PluginExecutionContext,
-    PluginInterface, PluginInterfaceManager, PluginSource, ServePluginError,
+    create_plugin_signature, get_signature, serve_plugin_io, EngineInterfaceManager, GetPlugin,
+    Interface, InterfaceManager, PersistentPlugin, PluginDeclaration,
+    PluginExecutionCommandContext, PluginExecutionContext, PluginInterface, PluginInterfaceManager,
+    PluginSource, ServePluginError,
 };
 #[doc(hidden)]
 pub use protocol::{PluginCustomValue, PluginInput, PluginOutput};