Update typings to support pub-sub of serialized messages

[wayneparrott]

646 describes the need to support publishing and subscribing to a serialized message topic. The TypeScript typings need to be updated to reflect the api changes.

Reviewing the code for #703 & #704, I have identified the following touch points:

1. Node#Options - add isRaw: boolean property
2. Node#SubscriptionCallback = (message: Message) => void
3. Publisher#publish(msg: Message)

Edit - deleted initial thoughts about SerializableMessage

I think by tightening up the following methods and type declarations using generic types we can support sending and receiving a Buffer as an alternative to a structured Message:

• Node#createPublisher()
• Node#createSubscriptions()
• Node#SubscriptionCallback

Example:

class Publisher<T extends MessageTypeClassName> extends Entity {
    publish(message: MessageType<T> | Buffer): void;
  }

Node#createPublisher<T extends MessageTypeClassName>(typeClass: T, topic: string, options?: Options): Publisher<T>

Node#createSubscription<T extends MessageTypeClassName>(
      typeClass: T,
      topic: string,
      options: Options,
      callback: SubscriptionCallback<T>
    ): Subscription;

type SubscriptionCallback<T extends MessageTypeClassName> = (message: MessageType<T> | Buffer) => void;

Looking for feedback. (@mattrichard, @koonpeng, ...)

[minggangw]

I have merged #703 & #704, so I think we could start the typescript part for the serialized messages, thanks!