[gRPC] Support streaming operations

[witemple-msft]

gRPC has a stream operator that is supported on both directions of the model:

Here's an example:

service ExampleService {
  // Unary
  rpc UnaryCall (ExampleRequest) returns (ExampleResponse);

  // Server streaming
  rpc StreamingFromServer (ExampleRequest) returns (stream ExampleResponse);

  // Client streaming
  rpc StreamingFromClient (stream ExampleRequest) returns (ExampleResponse);

  // Bi-directional streaming
  rpc StreamingBothWays (stream ExampleRequest) returns (stream ExampleResponse);
}

This issue tracks a design for supporting this in cadl-grpc when Cadl's streaming library support is available. See #154

[witemple-msft]

gRPC Priority: blocked
It's possible that we could do something gRPC-specific here, but better if we have core support in cadl for streaming operations.