[Content Fragment] Ability to restrict allowed CF models from template policy

As a template author who includes a Content Fragment in a template I currently have 2 options
* Allow the page author total flexibility to insert any CF they desire
* Lock the component to a specific CF in the DAM

I would like to allow the author the flexibility to choose whatever CF they desire, but restrict it to only those CF created from a specific CF Model. This means that I know the overall structure of the final pages created from this template but allow the actual content to change.

No files or tests are named in the issue. Start by tracing how the Content Fragment component reads template policies and selects allowed fragments, then determine how a policy could restrict choices by Content Fragment Model. Done means template authors can allow any fragment from a selected model while preserving the existing unrestricted and fixed-fragment options.

Written by the indexing model from the issue text.