`@rpc` support and examples

[StepanTheGreat]

Hello, I've been trying to find any way to use godot's RPCs in gdext, but couldn't find any example of it (searched for it in the issues and features under this repo as well). It would be great to see a working gdext example like in gdnative, or at least some code examples to see how to declare RPC functions? So far I was only able to find the Base rpc method, but that's about it.