Cherry-pick #20779 to 7.x: [Elastic Agent] Rename *ConfigChange to PolicyChange

[michalpristas]

Cherry-pick of PR #20779 to 7.x branch. Original message:

This rename the ConfigChange to PolicyChange for:

• The internal type for the actions.
• The handler
• The introspections

This depends on change on Kibana to return the new Type and the new
"Policy" key.

The kibana API need to be adjusted to send the right action type and the right fields.

What does this PR do?

Why is it important?

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

• Fixes: [Elastic Agent] Applies POLICY* Changes from Fleet to the Agent. #20687 20687

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #21390 updated]

• Start Time: 2020-09-30T08:56:53.161+0000

• Duration: 35 min 15 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1390
Skipped	4
Total	1394