es_out: support Upstream Servers with configuration overriding

[mabrarov]

Implementation of Upstream feature for the Elasticsearch output plugin.

Refer to fluent/fluent-bit#7608 for the changes in Fluent Bit code.

[github-actions]

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 fluent/fluent-bit#7608

[github-actions]

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.

[cnorris-cs]

I left suggestions, but I've added my technical writing-based approval based on that to keep this moving.

Primarily, I was looking to help the table be more consistent and readable, while implementing some style-based suggestions for consistency.

[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.