es_out: support Upstream Servers with configuration overriding

[mabrarov]

Implementation of Upstream feature for the Elasticsearch output plugin.

Refer to https://github.com/fluent/fluent-bit/pull/7608 for the changes in Fluent Bit code.

[github-actions[bot]]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[mabrarov]

This pull request is waiting for review of https://github.com/fluent/fluent-bit/pull/7608

[github-actions[bot]]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[mabrarov]

Hi @cnorris-cs,

I applied some (!) parts of your suggestions as well as introduced additional changes for consistency with your suggestions. Please take a look at this pull request one more time when you have time. Some of the changes are implemented in dedicated commits (for ease of revert in case of concerns) - please let me know if I need to squash all commits into a single one.

Thank you for your review.

[lockewritesdocs]

@fluent/fluent-bit-maintainers -- this PR needs a technical review.

[esmerel]

This still looks ok to me from a writing perspective.

[eschabell]

@esmerel can you look at the Vale check failures, I'm seeing this error before:

Raw Output: {"message": "[FluentBit.Headings] 'Classic mode Configuration File with Upstream' should use sentence-style capitalization.", "location": {"path": "pipeline/outputs/elasticsearch.md", "range": {"start": {"line": 163, "column": 6}}}, "severity": "INFO"} Error: reviewdog: Too many results (annotations) in diff. You may miss some annotations due to GitHub limitation for annotation created by logging command. Please check GitHub Actions log console to see all results.