What kind of change does this PR introduce?

The behavior to find the alg with which the id_token is signed is:

• use alg is set in get_jwt_config if defined
• else use client.id_token_signed_response_alg if defined
• else use the default RS256

I think client.id_token_signed_response_alg should take precedence over get_jwt_config, but that would be a breaking change. I will open another ticket for that, this could target v1.8.

Working on this part of the code made me realize that he get_jwt_config might be a little outdated. We would probably want use get_server_jwks instead here, instead of manually passing a key. What do you think?

fixes #755

Checklist

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• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.

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