feature: add possibility to specify JWKs key for authorizer

[lukedirtwalker]

A clear and concise description of what you want the system to do.

Currently we can use JWKs from an URL when using the IDP plugin. But to support the client credentials OAauth flow where the server would already get a token signed by someone else it would be helpful to support JWKS fetched from an URL.

The implementation could leverage https://github.com/MicahParks/keyfunc. I would probably extend the key_config source to have a value indicating that it should fetch the JWKs.

I would be happy to contribute this feature, if you tell me whether the what I propose roughly makes sense.