Allow token validation outside the axum middleware context

[Tockra]

I want to leverage Keycloak token validation in my Axum middleware and elsewhere in my application, like websocket authorization. Hence, I've centralized the validation logic in decode.rs and introduced a method in the Keycloak layer for easy JWT token validation with a string reference.

Regarding two updates, I'm uncertain about:

• Modifications to the Extra parameter in KeycloakService (service.rs, line 24).
• The impact of changes to KeycloakService fields on performance or thread safety. I believe there shouldn't be any issues, but I admit I may not fully grasp all design decisions behind axum-keycloak-auth.

Changes:

• Implemented main validation logic in decode.rs (extract_jwt_token function).
• Eliminated duplicate fields in KeycloakService, replacing them with a single instance of KeycloakLayer to streamline the code.
• Introduced a token validation method in KeycloakLayer to enable token validation outside the Keycloak middleware, enhancing flexibility.

[Tockra]

What do you think @lpotthast

[lpotthast]

Hey, back from a vacation. This looks really good! Thanks a lot for making these changes!