Client RPC configuration cleanup

This task tracks some cleanup work that needs to be done following the implementation of the "requestConfigFromFrame" configuration option to enable the client to fetch its configuration via an RPC to an embedder:

- [x] Document the "requestConfigFromFrame" option in the public docs
- [ ] Separate out the generic RPC server and specific method handling in `sidebar/cross-origin-rpc.js` and move the former into `sidebar/util/postmessage-json-rpc.js` and the latter into `sidebar/services/`.