Key management overhaul #856

The different modules ask developers to implement similar methods about key management

• methods to get the server JWKs (called get_jwks for rfc9068 and the coming rpinitiated , get_jwt_config for OIDC grants, resolve_private_key for OIDC UserInfo endpoint).
• methods to get server signing/verification algorithms (get_supported_algorithms for UserInfoEndpoint, get_server_registry for the coming rpinitiated)
• methods to get the client keys (resolve_client_public_key in rfc9101, resolve_client_public_key and resolve_client_key in rfc7523, resolve_public_key in rfc7591)
• methods to get the client signing algorithms (get_request_object_signing_algorithms in rfc9101)

I think that now that the joserfc has been merged, we should overhaul all of this to make thing clearer and easier:

• use the same method names everywhere
• attach server keys / algs management to the AuthorizationServer object, and use them as default (though this is not technically a rfc6749 feature, maybe this can be handled as a AuthorizationServer extension?)
• allow overloading of keys / algs in the different modules
• joserfc can easily extract a public key from a private key, so I am not sure that Authlib need to ask for both
• keep backward compatibility in 1.7 and display a deprecation warning for 1.8
• get_jwt_config especially feels odd