implement plugin command execution

- change .proto file for plugins
- add global usage guide for all commands
- handle commands not included in the built-in set

src/ssh.rs

@@ -14,7 +14,7 @@ impl Handler for ClientHandler {
 
     async fn check_server_key(
         &mut self,
-        server_public_key: &russh::keys::ssh_key::PublicKey,
+        _server_public_key: &russh::keys::ssh_key::PublicKey,
     ) -> Result<bool, Self::Error> {
         Ok(true)
     }