Bidirectional stream support in vBMC proxy

Adds a new verb to support bidirectional streams. The implementation is the same as for a subscribe stream, but plugins should also return an `EventHandler` of their own which is called with messages sent in to the plugin.

PiperOrigin-RevId: 700570241
Change-Id: I8fa18446f27a4a93099d98f3334fade9b1afd2d1
7 files changed