Add claims per client in id token

[nsklikas]

Adds the id_token_claims to clients, which is used to add specific claims per client to the id token.

This would give the ability to the clients to choose any set of claims they want when the registration endpoint is activated, but they would be able to do that nonetheless by using the claims parameter. Maybe we should add an option to limit the claims that can be included in an id token(e.g. "allowed_claims")?

[rohe]

I don't get the new test's name seems to me it's more about client claims enabled then client claims disabled. Or am I missing something ?

[nsklikas]

The enable_claims_per_client parameter defaults to None(False).
In the test_client_claims_disabled test I don't set it to anything so it defaults to None (ie claims per client are disabled).

Do you think I should change rename something or simply add self.endpoint_context.idtoken.enable_claims_per_client = False to the last test for clarity?

[rohe]

Maybe not explicitly set it to False but at least there should be a comment about what the default is.

[nsklikas]

Ok