Adds the ability to use different payloads (#710)

Motivation:

gRPC should be agnostic to the type of payload that it sends to the peer to allow for other message types to be used (e.g. Flatbuffers).

Modifications:

- Make the generated code generic over `GRPCPayload`
- Provide a default implementation of `GRPCPayload` for `SwiftProtobuf.Message` in `GRPCProtobufPayload`
- Update generated code such that request and response types conform to `GRPCProtobufPayload`
- Shuffle around some of the server code to minimise payload copies

Result:

gRPC can send/receive payloads which conform to `GRPCPayload` instead of `SwiftProtobuf.Message`

.gitignore

@@ -3,6 +3,7 @@ project.xcworkspace
 xcuserdata
 DerivedData/
 .build
+.swiftpm
 build
 /protoc-gen-swift
 /protoc-gen-grpc-swift