Support socket control messages for rpc calls

I'd like to be able to have rpc's that can pass file descriptors over transports which support it.

Today there is some support for an interceptor with a custom handshaker to pass along unix rights messages on connection initiation.
The method of passing file descriptors is similar (same mechanism, different message).

The difference here is it would need to be supported per request instead of at connection time (I think), so I'm not sure we'd have access to a handshaker.

Start by reviewing the existing interceptor and custom handshaker support for Unix rights messages, then trace how transport messages are handled for individual RPC requests. Define how per-request file-descriptor control messages should work across supported transports; completion requires an agreed design and corresponding implementation.

Written by the indexing model from the issue text.