RPC: Refactor code to allow non-serialized non-RPC-invocable RPC methods

As discussed at !2143 (comment 3029444) we may want to add a new kind of method to replace ClientConnectionTarget.

This kind of method would be:

• Invocable on rpc::Objects
• Not directly accesible from the RPC session (and therefore it would have no method name)
• Able to take arguments and return results that were not (de)serializable.

It's not IMO a 100% sure thing that this is how we will want do go; IMO we should gain experience with our RPC system to see if this will deliver benefits. Also see #1401 (closed); we would want #1401 (closed) if we do this at all.