Add Metadata to Secrets

This blueprint proposes adding a metadata property to secrets.

The Barbican API returns metadata to the user about secrets, but the user is not allowed to set metadata options that are not supported by Barbican. This blueprint proposes adding a metadata field to secrets that allows users to store any metadata with a secret.

Note that this blueprint only provides the model and backend storage changes required to store metadata. It does not provide a change to the interface to allow the user to specify new metadata with the key. That is reserved for another blueprint.