Implement must match section for custom metadata

[mnm678]

Request for New Functionality

Description of the Desired Feature / Behavior

The Uptane Standard describes an optional must match field in the custom field of Targets metadata that must match on the Director and Image repositories. This section would include the release counter and hardware id. This field is optional in the standard, but improves security when the Director is compromised by preventing roll back attacks and installing an incompatible image.

This can be implemented after #5 which adds release counters and hardware ids to the implementation.