[#3519631] Add a deleteTemporaryCredentials() function, so cached credentials...

[#3519631] Add a deleteTemporaryCredentials() function, so cached credentials can be manually deleted.

This is important during multi-hour drush runs, as the keyvalue.expire store checks expiry against REQUEST_TIME which is set at the start of a run and never changes, so will never see a cached set of credentials as expired.

Closes #3519631