feat: Add async support to InMemoryBM25Retriever and InMemoryEmbeddingRetriever

[shadeMe]

Related Issues

• Async Pipelines haystack#6012

Proposed Changes:

Add async support to InMemoryBM25Retriever and InMemoryEmbeddingRetriever. And a fly-by doc fix.

How did you test it?

Integration tests

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 unit tests and updated the docstrings
• 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

[coveralls]

Pull Request Test Coverage Report for Build 11973374927

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 81.929%

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Files with Coverage Reduction	New Missed Lines	%
components/tools/openapi/_schema_conversion.py	3	88.79%

Totals
Change from base Build 11963918591:	0.03%
Covered Lines:	2344
Relevant Lines:	2861

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💛 - Coveralls

[shadeMe]

Failing integration tests are unrelated.