Support subprotocols without synchronized sequence ID

[PMalte]

I want to use this library with a subprotocol where the client and server have their own sequence ID. Both peers should be able to send requests and receive corresponding responses.

The problem I encounter:
The subprotocol is based on request-response pattern.
Imagine both peers use their own sequenceId to number their requests and both peers start with sequenceId = 1.
So it is possible, that both peers send a request with the same sequence ID at the same time. Now the client considers the request of the server as the response of its request and the real response of the server is dismissed.

This happens because the assignment of messages is only based on the sequence ID.

Feature Request:
Give the caller the opportunity to save additional data to every request so the assignment of messages is not only based on the sequence ID but on additional data.

Example:

Request opcode	Possible response opcodes
login	ack, nak
getProperties	properties, nak
...	...

If I send a login command and it gets the sequenceId = 34, I can save the possible response opcodes as additional data for that request and when I get a response matched by the sequenceId I can validate the response by checking if the response opcode matches ack or nak.

const wsp = new WebSocketAsPromised(wsUrl, {
  packMessage: data => JSON.stringify(data),
  unpackMessage: data => JSON.parse(data),
  attachRequestId: (data, requestId) => Object.assign({id: requestId}, data), // attach requestId to message as `id` field
  extractRequestId: data => data && data.id,                                  // read requestId from message `id` field
  isPossibleResponse: (data, additionalData) => additionalData.possibleResponses.includes(data.opcode) // <--- new parameter
});

wsp.sendRequest({foo: 'bar'},{requestId: 42, additionalData: {possibleResponses: ['ack', 'nak']}});

I worked a bit on this idea in the following fork: https://github.com/PMalte/websocket-as-promised/tree/feature/bidirectional-subprotocols