get_credential checks for active session or group membership

[hongsudt]

We discussed on slack adding something like get_credentials(hostname, require_session=True) which raises an exception if the credentials do not allow a probe to GET /authn/session to find an active session. This sort of test would also be needed for the fallback scenario, since it is not enough to just pull a (possibly stale) cookie or token out of the local storage and declare success.

For some scripting, it might also be useful to require a user be in certain groups? Or maybe you want to do a different requirement check further along like checking that you have a certain "right" on an ermrest catalog?

Note: we also discussed to check oauth-based credentials first because those are potentially renewable.