Multi-Container: Build an initial demo

[eddyashton]

Most of the work for this is covered by other tickets, but using this as a hold-all for remaining work towards an MVP demo.

Additional work I see for this demo specifically:

• Writing a simple executor, and some infra help to operate it
• Breaks from our intended gRPC API for basic gRPC compatibility (ie - no streaming responses means polling requests, no long-lived gRPC blocking server calls means regular Ready polling by clients)
• Timers and timeouts for pending client requests, to check that additional error handling can be added in future