airframe-grpc: Streaming interface

[xerial]

Design questions:

• Should we use StreamObserver? It has no control for the back-pressure. Related discussion can be found in Simplify implementation of back-pressure in StreamObserver-based stub grpc/grpc-java#1549
• What interface should we use? Stream[X] is already in Scala collection, so using it can be confusing. => We can use Rx[X]
• grpc-kotlin is using Flow with coroutine
• Using scala-async can be an option?

Related:

• https://github.com/salesforce/reactive-grpc
• https://github.com/fiadliel/fs2-grpc