Add support for google secrets manager secrets

Logan can currently decrypt KMS encrypted secrets in the directory but our future plans call for using Google Secrets Manager to store credentials. See: https://docs.google.com/document/d/1fd0M6hL9fpTP12hFFOw2CTCw21U_1M6C2JmreyUV-ws/edit#

Add support to logan to decrypt SM secrets.

placeholder from https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/g-suite-authenticator/issues/1