feat: allow filtering documents on all fields (v2)

[ZanSara]

Related Issues

• fixes Improve MemoryDocumentStore filtering to ease retrieval #4838

Proposed Changes:

• To implement BM25 retrieval in MemoryDocumentStore we need to be able to filter out non-text documents
• This usecase might be really handy in many other situations, therefore I decided to extend the filtering
• This PR allows to filter documents by metadata as before, plus by any other Document field. For example:

docstore.filter_documents(filters = {
    'some_metadata_key': 'some metadata value',
    'content_type': 'text'   # This is the new feature
})

How did you test it?

• Locally run the unit tests
• Added many more to the base suite

Notes for the reviewer

Related to #4768, but it's not necessary to wait for it.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[masci]

Left a couple of comments but overall looks good to me. For the sake of being an old broken record, I would have probably shipped the safe operators and the docs flattening in two separated PRs 😄

[ZanSara]

For the sake of being an old broken record, I would have probably shipped the safe operators and the docs flattening in two separated PRs smile

Ugh good point... I didn't realize we needed the safe operators until I tested on flat Documents, but in hindsight that's true. Thanks for the reminder! One PR at a time I'll learn to notice 😄

[coveralls]

Coverage: 37.158% (-0.2%) from 37.336% when pulling 0808a96 on v2-memory-extended-filters into 9cb153d on main.

[masci]

LGTM!