feat: async API when `Response` has been processed. (#1281)

* feat: add `RpcModule::register_raw_method`

* add proc macro support

* rename API

* simplify API with MethodResponse::notify_when_sent

* improve notify API

* fix nits

* introduce ResponsePayloadV2

* impl ResponsePayloadV2 for T

* cleanup

* client: proc macro support for custom ret_ty

* add tests

* address grumbles

* remove unused code

* fix tests

* proc: revert unrelated changes

* remove panics; move should be enough

* bring back UI tests

* grumbles: remove NotifiedError

* break stuff for uniform API

* make more stuff private

* remove ResponseErrorUnit type alias

* fix ui tests

* Update proc-macros/src/render_server.rs

* Rename ws_notify_on_method_answered.rs to response_payload_notify_on_response.rs

* remove unit_error APIs

* replace notify_on_x with notify_on_completion

* Update server/src/transport/ws.rs