Add sidechannel proxying capability to Praefect
This is a follow-up to #1216 (closed).
- Configure Praefect front-end to accept backchannel connections (it currently does not)
- Create gRPC client middleware that can proxy sidechannel connections
- Integrate this client middleware into Praefect backend gRPC connections so that it is used on all backend gRPC calls going out of Praefect