Support GRPC upstream servers

[chirino]

Support GRPC upstream servers similar to how Openapi servers are supported now. Implement auto conversion to a GraphQL schema if you have it's service definitions.

[wtrocki]

This is more complex issue, however it will enable Knative integration

[namit-chandwani]

Hey, I'm learning gRPC right now while implementing it in a project.
Will start working on this issue in a couple of days, once I'm ready.
In case I get stuck at any step, I will discuss it over here.
Thanks!

[wtrocki]

@namit-chandwani real issue is to replace graphql to upstream implementation. Adding GRPC will not be that simple until we clean up things and setup proper architecture

[namit-chandwani]

@wtrocki Got it. I'll start working on that issue first. Thanks!