ID Token encryption

[vernans]

First of all thank you for publishing this package, it's making my life a whole lot easier.

I noticed a feature missing that's kind of critical for what I'm doing: The idpyoidc.client.oidc.access_token.AccessToken service does not support encrypted ID Tokens. I think I'll manage to implement it (providing a new Service + Response type and adapting the verification of the Response and the post hook for the response parsing in the service I hope) but it would certainly be nice if the classes would support it on their own.

If you're interested I'm happy to provide an implementation (with a little guidance) if you're not planning to implement this soon.