Added an easy to use WebSocket handler AsyncWebSocketMessageHandler with an example

[mathieucarbou]

This PR is a proposal to add an easier to use WebSocket handler for unfragmented messages, which is 90% of the use cases. The current WebSocket API is complex to use, I think this one will help a lot.

[vortigont]

that's nice indeed! It's not common to implement reassembly logic for incoming messages for users.
One though on this - maybe have one more error callback to let user know that fragmented message has been received/discarded?

[mathieucarbou]

that's nice indeed! It's not common to implement reassembly logic for incoming messages for users. One though on this - maybe have one more error callback to let user know that fragmented message has been received/discarded?

Ah yes, good idea!
Actually, if final is false and there is not 1 frame, I could add a callback that will received all fragments... Will improve. This why, this will be a generic solution for everybody.