[protocol] Allow for "reverse endpoints"?

[jamesmunns]

Since we have symmetry for Topics, do we want to have the ability for servers to make endpoint requests to clients? This could be optional (not all servers must support it), and could tie into #37 as we'd need a client-like interface.

This would allow for things like asking poststation to proxy requests to other clients, as well as transactional operations from server to client.

This was asked in chat, and was also a potential feature requested in a discussion about rust-analyzer.

I am open to sponsorship to investigate this as an option for both postcard-rpc and poststation.