Secret-service: missing support for unlocking with a known secret (without a prompt)

The current API does not support unlocking the secrets DB with a known secret without a prompt. As a result, PAM modules that are used to unlock the secrets DB have to bypass the DBus API in a provider-specific manner. This might also have implications for separation of the provider-specific service backend from their frontend interface.

It may be desirable to extend the API to allow the Unlock() method or a variant thereof to receive a known secret with which the DB can be unlocked.