Support MQTT over websocket

Currently, the client library we are using can connect with websocket. However, we are missing a way to properly either describe it in TD or implement it in core.

To describe, we can:
- `ws` scheme +.`subprotocol: mqtt`
- `mqtt+ws` (not recommended, but we can handle it): other examples coap+ws, modbus+tcp

To implement:
- We lack support to register multiple schemes. We cannot register `mqtt` and `mqtt+ws`
- Currently subprotocol is handled by the main client protocol. So it means that if we go for `ws` URI scheme + `subprotocol : mqtt`, the code handling mqtt should be copied or imported to the `websocket-binding`, which is not the right way forward. Then we need a way to register combos of URI+subprotocol.