Support async for WebSocketController

**Is your feature request related to a problem? Please describe.**
I checked the [FunctionTraits.h](https://github.com/drogonframework/drogon/blob/master/lib/inc/drogon/utils/FunctionTraits.h) code, and noticed there are no traits to add WebSocketController children so it supports Task<> returns and make them coroutines like HttpController.

Is that possible?

**Describe the solution you'd like**
Update the traits so WebSocketController supports coroutines and be made async like HttpController.

**Describe alternatives you've considered**
I've found no alternatives inside the library.

**Additional context**
I leverage coroutines and use all asynchronous endpoints, but as I start using WebSockets for VelocityPack message streaming, I noticed it's blocking, and I would prefer to have all my drogon workers not get constantly blocked.