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Description: This will create a feature request which anybody can view and comment on.


Please describe your requested enhancement. Good feature requests will solve common problems or enable new use cases.

According to the docs on IAP
payloads(https://cloud.google.com/iap/docs/signed-headers-howto#verifying_the_jwt_payload),
we can expect a `google` key that contains access levels and device id
info. However, the `TokenPayload` interface does not expose this, and so we
can't fetch info from `google` without casting the payload


What you would like to accomplish: google` key that contains access levels and device id info located PayloadToken type in node js auth lib

How this might work:
- Define for the Google key in the payload
- Include a nullable `google` attribute of the above type in the
`TokenPayload` interface


If applicable, reasons why alternative solutions are not sufficient: n/a

Other information (workarounds you have tried, documentation consulted, etc):

https://cloud.google.com/iap/docs/signed-headers-howto#verifying_the_jwt_payload