x-pack/filebeat/input/httpjson: backport configurable keep-alives

[efd6]

What does this PR do?

This is a manual backport of #34014 tailored to 7.17.

Keep-alive options are provided for both v1 and v2 HTTPJSON, so to simplify the change the keep-alive logic is put in v2 and exported for use by v1.

Why is it important?

Customer request.

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.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates x-pack/filebeat/input/{cel,httpjson}: make transport keep-alives configurable #34014
• Relates x-pack/filebeat/input/{cel,httpjson}: make DisableKeepAlives configurable #33951

Use cases

Screenshots

Logs

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• Start Time: 2023-03-06T22:21:32.766+0000

• Duration: 70 min 38 sec

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Test	Results
Failed	0
Passed	3449
Skipped	305
Total	3754

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🤖 GitHub comments

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[andrewkroh]

LGTM