feat: extract label aggregation

[tstadel]

Related Issues

Label aggregation is encapsulated into BaseDocumentStore's get_all_labels_aggregate and cannot be used outside the document store. Label aggregation should not be tied to DocumentStores.

Proposed Changes:

• extract logic into method aggregate_labels of new module haystack.utils.labels
• refactor
• adjust docstrings

How it's used:

from haystack.utils import aggregate_labels

labels = fetch_labels()  # from anywhere, returns List[Label]
aggregated_labels = aggregate_labels(labels)  # returns List[MultiLabel]

How did you test it?

• rely on existing tests of BaseDocumentStore.get_all_labels_aggregated()

Notes for the reviewer

• None

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