Add support for custom WebSocket path

[racknoris]

Add support for custom WebSocket path via optional websocketPath property.
Allows connecting to brokers that serve MQTT over WebSocket on non-root paths (e.g., /mqtt).
Tested with wss://

[shamblett]

Thanks, good addition, Is there any reason you know why this can't be added to the browser client?

[racknoris]

You're welcome, hope it will help others as well.
I should dig deeper into the mqtt concept to answer properly because I saw the issue with constant path in the Uri, so it seemed appropriate to transfer an internal path and "inject" it into the class where needed. I believe it is safe to assume flutter web will behave the same, so I'll make the changes in the browser section and test against "ws://" but it will take some time to get to that.

Thank you for the quick merge and response

[shamblett]

Great thanks, I only ask because the path is part of the Uri, not a specific option on the web socket like timeouts for instance, the Uri is common to both so I think should be OK. Also, its only an option, it doesn't exist at the moment and in the future if someone uses it and we hit problems we can address it then. The client is not forcing this on anybody.

No problem with the timescale, please note you now have a branch for this named pr607 no need to use master.

[racknoris]

Ok I understand