feat: Add support for async in `DocumentStore`

[shadeMe]

Related Issues

• https://github.com/deepset-ai/haystack/issues/6012

Proposed Changes:

This PR adds async variants of methods in the DocumentStore protocol and implements them for the OpenSearchDocumentStore.

How did you test it?

Integration tests

Notes for the reviewer

This new tests added in this PR require a running OpenSearch backend. This is currently not set-up in the CI, so you'll need to test it locally (refer to the testing instructions in the OpenSearch integration's readMe).

Also, the tests require haystack@main until 2.6.0's release.

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