chore(elasticsearch sink): Add documentation and tests about OpenSearch

[taisho6339]

Summary

While vector supports OpenSearch in code, it isn't described enough. Also, there are no tests and documentation. With this PR, I added unit tests and the explanation in the doc.
As a follow-up PR, an integration test for OpenSearch should be added.

Issue: #16761

Vector configuration

How did you test this PR?

• make test
• make test-docs

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

• Please read our Vector contributor resources.
• Do not hesitate to use @vectordotdev/vector to reach out to us regarding this PR.
• Some CI checks run only after we manually approve them.
  • We recommend adding a pre-push hook, please see this template.
  • Alternatively, we recommend running the following locally before pushing to the remote branch:
    • cargo fmt --all
    • cargo clippy --workspace --all-targets -- -D warnings
    • cargo nextest run --workspace (alternatively, you can run cargo test --all)
• After a review is requested, please avoid force pushes to help us review incrementally.
  • Feel free to push as many commits as you want. They will be squashed into one before merging.
  • For example, you can run git merge origin master and git push.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run cargo vdev build licenses to regenerate the license inventory and commit the changes (if any). More details here.

[drichards-87]

Thanks for submitting the PR. It looks good from a Docs perspective. I'll leave the technical review to the Vector team.

[pront]

Thanks!

[taisho6339]

@drichards-87 @pront Thanks for your review! Apologies for missing cargo vdev fmt. Could you approve the workflows and do quick check again?