Native RPC prototype

[bcherry]

This is a prototype implementation of the native RPC spec . I've made a few tweaks to the original proposal - notes contained herein.

This is just a prototype - it isn't suitable to release. I built it with user packets and specific topics, but it let me set up an example use case and iterate the user-facing interface until it felt right. I feel good about this version.

Once I've collected some feedback I'll finish the protocol, update the SFU to route them, implement the core pieces in rust, then finish this PR out. Then we've got to do it in all the other SDKs, too....

Feedback appreciated on the API - see the example for a sense of how it works. I feel good about this but there's always room for improvement.