feat: ability to map custom claims

[OskarsPakers]

Ability to specify additional claims to be mapped to id_token for hydra.

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[aeneasr]

Thank you for the contribution! Unfortunately this probably isn't a medium term solution, because the most important part when forwarding the traits is also checking the scope. For example, if the user allows scope phone then we can include the phone number - otherwise we can't.

Therefore this needs a lot of thought to be designed well. We'll probably come up with a solution in Ory Network at some point.

Since this repository really is more an example implementation, I think it's ok if people write their own logic matching their requirements here instead of trying to provide a generalized solution.