This is the start of a stack addressing ENG-8421.

The idea is that communication will occur between peer devices via this new message type, communicated across usePeerToPeerMessageHandler via Tunnelbroker.

We might also process these on the sending client.

I haven't tested this stack outside of Flow. I'd like to propose that we land this so the team can iterate. I'll make sure to test all of the functionality once it's integrated into the rest of the codebase.