iat selective disclosable

[fmarino-ipzs]

according to the latest version of the specification, iat is now selective disclosable. In generate.py this is given as default claim, making selective disclosure of iat not possible.

 if include_default_claims:
     claims = {
          "iss": settings["identifiers"]["issuer"],
          "iat": settings["iat"],
          "exp": settings["exp"],
     }

It should become:

 if include_default_claims:
     claims = {
          "iss": settings["identifiers"]["issuer"],
          "exp": settings["exp"],
     }

In addition, in the master settings.yml iat should be removed and should be added in the specification.yml file within the specific example folder.