Add reference for the transport key

Transport keys are used to ensure that the secret is pre-encrypted in such a way that only the client and the back-end store can decrypt the secret. This is for users which do not trust Barbican, but do trust the back-end secret store. Or for those that need FIPS and CC certified components.

Currently, the client gets the transport key from Barbican. But if the client does not trust Barbican, this is a potential vulnerability. We need to add the ability for the client to retrieve the transport key from the back-end store directly.