Add basic serverless support for elasticsearch, beats index management

[fearful-symmetry]

Proposed commit message

The tests that are a part of elastic/elastic-agent#3258 will pass with this PR.

There's a few parts to this PR:

• Adds an IsServerless() method to the ES client
• Changes the Ping() method in the ES client
• Adds some checks to the index management setup process to bypass ILM setup under serverless
• Adds some doc comments where needed
• Changes a handful of ES interfaces to support the IsServerless() method

If there's anything weird in this PR, it's probably because I spent a day just sorting through ancient ILM code.

So, I'm not sure if we want to merge this without DSL support, or add DSL support in a follow-up PR, or merge elastic/elastic-agent#3258 before or after this PR. Currently open to suggestions.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-09-20T16:36:30.074+0000

• Duration: 101 min 21 sec

Test stats 🧪

Test	Results
Failed	0
Passed	28277
Skipped	2013
Total	30290

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Tests succeeded.

🤖 GitHub comments

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
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git fetch upstream
git checkout -b serverless-aware upstream/serverless-aware
git merge upstream/main
git push upstream serverless-aware

[faec]

Looks good as far as the code here (thank you for adding more docs!)... I don't have the context to say much about inter-repo order of operations for this feature but it looks to me like merging this one wouldn't break anything, so it's probably a reasonable incremental step?

[faec]

Can this ever actually change, or is this just defensive programming? If the latter can you add a comment saying something like "this should already be false but let's be extra careful"?

[faec]

typo (indicies)

[cmacknz]

A few suggestions to improve error text to use official names, otherwise looks good.