Integrate with DOG to support crosswalks

[WillemElbers]

Implement crosswalks to allow conversion of metadata resources into a common denominator (exact schema to be defined, maybedc?) which we can use to extract basic information to prefill in the UI.

Ensures MSCR integration via clarin-eric/DOGapp#11

When a user adds a link (PID or URL) to a collection we ask some metadata (title, description, ...). By crosswalking the metadata record pointed to by this URL to some common denominator we can extract information to prefill in the title and description fields. The user just has to save or add the next resource. Advantages: less work for the user, more metadata that we collect in our collections.

• Call the DOG to query available crosswalks for a metadata resource. Use media-type and or schema information (maybe from the media-type)
• If a crosswalk is available, call the DOG to run the crosswalk and return the result.
• Use the result to prefill the relevant UI fields.