Cherry-pick #24907 to 7.x: Restart process on output change

[michalpristas]

Cherry-pick of PR #24907 to 7.x branch. Original message:

What does this PR do?

This PR adds an option to specify in a spec file behavior on output change as sometimes new output definition is not reloaded correctly on beats side.

Endpoint reloads output ok, fleet server does not reload output at all but we agreed to fix it on server side.

Why is it important?

#23596

#24538

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.

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Build stats

• Build Cause: Pull request #25160 opened

• Start Time: 2021-04-20T07:27:34.959+0000

• Duration: 57 min 47 sec

• Commit: 1a92f2b

Test stats 🧪

Test	Results
Failed	0
Passed	6740
Skipped	24
Total	6764

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Test stats 🧪

Test	Results
Failed	0
Passed	6740
Skipped	24
Total	6764