Implement custom token extractor

[blablacio]

I've been working on a websocket API for some time and just realized that browsers still don't support setting headers like Authorization (or anything else other than Sec-WebSocket-Protocol), which makes things a little bit more complicated.

So, now I'm wondering what would be the most straightforward and flexible way to allow for custom token extraction?

I was thinking that probably adding a new token_extractor field on Authorizer and AuthorizerBuilder along with a method on the builder to allow passing an extractor function would be the easiest way to extend token extraction without touching the JwtSource enum.

Might be a good idea to skip setting the token_extractor entirely if jwt_source is passed, but not sure about that.

Let me know what you think and if there's a better way to go about this.