Store tokens and other credentials across check runs

Any check that requires auth / credentials that expire needs to fetch these credentials on every check run. While this is the safest way to make sure checks are authenticated, it creates friction:

- extra runtime in a browser check or setup script.
- extra load on any authentication endpoint.
- waste of general cloud resource.

A user should be able to store tokens securely in a check run and fetch or revalidate the credentials when or before they expire.