Use example `FileTypes` to validate `Extractor` entries

[PeterKraus]

Originally in marda-alliance/metadata_extractors_registry#34:

• Make sure the URLs are resolvable through Fly (probably pointing to GitHub raw links)
• Add the files as examples in the file types models and expose them for validation
• Validate entries in the data folder and make sure they correspond to registered file types

This would mean that a CI in the yard repo would use the beam package to check whether the files in the marda_registry/data/lfs folder can be matched to FileTypes and processed using any matching Extractors in the yard.

• The obvious issue right now is how to deal with GitHub's LFS restrictions.
• The obvious issue down the road is how to deal with the server load.

[PeterKraus]

A possible suggestion:

Committing an example file for everything under the sun into this repo directly is probably not the best way forward. We should have a mechanism for providing persistent links to example files (e.g., archived files with DOIs) that the registry can download and use as test data. Probably this ends up being a registry of example files too, in that case...

See marda-alliance/metadata_extractors_registry#66