feat: add index parameter to TfidfRetriever

[anakin87]

Related Issues

• fixes Feature Request: Add index parameter to TFiDF retriever #1634

Proposed Changes:

TfidfRetriever automatically uses the default index of the Document Store.
You can't set/change index, eg for evaluation purposes.

This PR is just a first draft to implement this feature by minimally changing the node.

How did you test it?

CI;
added a new (improvable) test.

Notes for the reviewer

• assert isinstance(index, str) is necessary for mypy to not complain about the Optional nature of the index parameter
• I also improved test_retrieval_without_filters to skip an unnecessary check

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 the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[anakin87]

@ZanSara feel free to take a look 🙂

[ZanSara]

Looks good to me! Left only a couple of comments

[agnieszka-m]

Just a couple of language suggestions to align with our voice and tone :)

[ZanSara]

LGTM! 🚀

addressed