Implement websocket functionality

[DeBelserArne]

Is your feature request related to a problem? Please describe.
Not really a problem, more like having a stream of data. For example, listening to filled orders. Right now we would have to use the REST API with an interval to poll the data.

Describe the solution you'd like
If possible for you implement websockets:
https://developer-pro.bitmart.com/en/ws/

Describe alternatives you've considered
Not really considered alternatives.

Additional context
I have tried this library, but it seems a bit outdated. The request which tries to authenticate always returns a 415 "Wrong Media".
https://github.com/quadency/quad-bitmart-client

Let me know what you think!

I've tried implementing it myself, but I got stuck on the "sign" part:
https://developer-pro.bitmart.com/en/ws/spot_ws/login.html