Added interface to wrap ObsWebsocket implementation

[grofit]

This allows downstream consumers to mock the object easier, also makes it clearer in one place what the implementation exposes due to the partials splitting it out lots.

If you don't want it no problem, only downside of having it is that if you update any sigs in the implementation they also need to be updated in the interface, but tbh with resharper or rider its a 2 second job to export interface => select public and it would regenerate.

[grofit]

I mentioned this on #99 I think, so its optional but it may help others given it puts a proxyable interface between everything for AOP/Mocking purposes.