Closes#3

• Adds protobuf code generation
• Adds utilities and wrapping code around RPC message types
• Adds RPC protocol code required for sending and receiving messages
• Adds a end-to-end test with messages and replies between two Speakers

TODO:

• Add thecsharp_namespace option to the upstreamvpn.proto
• Decide between event delegates (callbacks) or channels for receiving messages
• PlumbCancellationTokenSource into more operations (any asyncSpeaker operation should use the suppliedCancellationToken as well as the globalCancellationTokenSource)
• Make the startup process async?
• Try to declutter the generic type specifications around the codebase
  • It may be possible to condense the twoTS/TSi andTR/TRi into justTS andTR with some finangling and changes around theRpcMessage<T> type
    • Maybe switch (back) to an interface rather than an abstract class
• Move the role to be attached to the type so it doesn't need to be passed in manually to theSpeaker at all
• Tests
  • ApiVersion
  • RpcRole
  • RpcHeader
  • RpcMessage (and RpcRoleAttribute)
  • Serdes
  • Speaker