Cherry-pick #22690 to 7.x: Log level reloadable from fleet

[michalpristas]

Cherry-pick of PR #22690 to 7.x branch. Original message:

What does this PR do?

This PR introduces log level reloading initiated by SETTINGS action coming from fleet introduced in this PR: elastic/kibana#83707

This strategy reexecs agent on log level change.

Why is it important?

#20756

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.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #22729 opened]

• Start Time: 2020-11-24T10:02:18.752+0000

• Duration: 57 min 39 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16424
Skipped	1385
Total	17809

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	16424
Skipped	1385
Total	17809