AIPscan is missing API endpoints for listing the file formats and PUIDs in a Storage Service

[tw4l]

PR #76 introduces new API endpoints that accept a file format name or PUID as input, and return a list of all of the AIPs with that given file format or version, along with the count and total size of the matching files for each AIP.

It's not currently possible via the API alone to discover which file formats or PUIDs can be found within a Storage Service. Ideally we would want API users to be able to do something like the following (as described by @ross-spencer in coder review for #76):

• Query format name groups
• Query for the report-like statistics based on that filter

Supporting such an API-first workflow would require adding new endpoints that take a storage_service_id as input and list the available file formats or PUIDs found in that Storage Service. The StorageService model has some properties introduced in #76 such as StorageService.unique_original_file_formats which should make implementing such an endpoint relatively painless.