feat: OpenSearch - async support

[anakin87]

Related Issues

• fixes add run_async to opensearch retrievers #1392

Proposed Changes:

• Async support for OpenSearchDocumentStore and its retrievers.
• This is mostly a migration of work done in feat: Add support for async in DocumentStore haystack-experimental#96 and other PRs approved in experimental.
• I took care of fixing some errors and integrating async with small features/changes merged in this integration and absent in the original experiment.
• (I also took the opportunity to adopt Haystack logging in OpenSearch - see Use Haystack logging across integrations #1409)

How did you test it?

CI; migrated many new tests.

Notes for the reviewer

Even if this is a large PR, this is not a breaking change (original tests pass without requiring significant modifications).

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:.

[mpangrazzi]

Looks good!