Control bridge gains a generic 'shared control request' handler

What

Remote Control's connection options previously had a specific onGetUsage callback just for usage queries. This has been replaced with a more general answerSharedControlRequest callback that can handle more kinds of requests.

• The remote-bridge connection options now expose answerSharedControlRequest in place of the older onGetUsage callback.
• Before the control bridge dispatches a control_request to its specific handler (for things like initialize or set_model), it first calls answerSharedControlRequest on the request. If that returns a promise, the request is handled asynchronously; if it returns a result right away, a control_response is sent immediately without running the normal per-type handling.

Why

This generalizes what used to be a single-purpose usage-query callback into a reusable path for answering various control requests, making it easier to add new request types without special-casing each one in the dispatch logic.