Consider adding fastwebsockets as an option

[AdrianEddy]

Is your feature request related to a problem? Please describe.
It would be nice to have an option to select fastwebsockets as a feature instead of tungstenite

Describe the solution you'd like
Add fastwebsockets alternative to tungstenite as a cargo feature

Describe alternatives you've considered
I could use hyper directly with fastwebsockets but salvo's interface is much nicer

[chrislearn]

I feel like adding fastwebsockets may make the default websocket implementation more complex.

The types of fastwebsockets and tungstenite should not be universal, which will lead to completely different interfaces exposed to the public by default. This is not as good as externally implementing a separate middleware to support fastwebsockets.

[AdrianEddy]

got it, would it maybe make sense to add it to examples/ with the usage as a middleware?