Add GetSecret() to allow direct access to passphrases without KDF and
wrapping by a DEKStore.
This will be used by fscrypt, which has its own KDF and wrapping. It
will allow users to take a k8s secret, for example, and use that
directly as a password in fscrypt.
Signed-off-by: Marcel Lauhoff <marcel.lauhoff@suse.com>