Support for JWT handling

Hi ! First of all, thank you for your amazing work !
I'm planning on adding support for JWT to the `actix-web-httpauth` crate.
Is this the right place to implement such feature ? Or would it be in another crate ?

Like for the `Bearer` auth already implemented, the `JWT` auth would have a config with different settings on how to validate the JWT and a `struct` wrapped in an `Arc>` in which to store, access and hot-replace the `JWKS`.

An extractor would also be available to extract common header fields as well as common claims. (Maybe find a way to make the claims struct dynamic, so that people could plug-in there own struct for the claims)

This features would require the following crates :
- jsonwebtoken
- serde
- serde_json
- chrono