Problem: get package metadata endpoint needs pagination

[jrwdunham]

A request of the form GET /api/v2/file/metadata/ will load all of the File model instances belonging to a package into memory and serialize the resulting list to JSON and return that JSON in the response. For large AIPs this will inevitably cause timeouts for clients. This "metadata" (array of File objects) should be paginated.

Better yet, it should be accomplished by filtering for the SIP using a search-enabled files/ (over the true "File" model/resource) endpoint that has pagination built in, cf. https://github.com/artefactual/archivematica-storage-service/blob/2c4f950780f06caf231d1e2a9a3b97ff9c887f61/storage_service/locations/api/beta/__init__.py

Connected to #369