Support for getting help text from a plugin command (#12243)

# Description
There wasn't really a good way to implement a command group style (e.g.
`from`, `query`, etc.) command in the past that just returns the help
text even if `--help` is not passed. This adds a new engine call that
just does that.

This is actually something I ran into before when developing the dbus
plugin, so it's nice to fix it.

# User-Facing Changes


# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] Document `GetHelp` engine call in proto

crates/nu_plugin_example/src/commands/main.rs

@@ -28,11 +28,10 @@ particularly useful.
     fn run(
         &self,
         _plugin: &Self::Plugin,
-        _engine: &EngineInterface,
+        engine: &EngineInterface,
         call: &EvaluatedCall,
         _input: &Value,
     ) -> Result<Value, LabeledError> {
-        Err(LabeledError::new("No subcommand provided")
-            .with_label("add --help to see a list of subcommands", call.head))
+        Ok(Value::string(engine.get_help()?, call.head))
     }
 }