Cherry-pick #26275 to 7.13: Enable agent to send custom headers to kibana/ES

[michalpristas]

Cherry-pick of PR #26275 to 7.13 branch. Original message:

What does this PR do?

This PR modifies container and enroll commands to accept list of headers which are then sent to kibana/ES when fleet-server or metric/file beat send events or performs setup.

Headers are hold in memory and are added to ES output in opt-in manner using inject_custom_headers rules in a spec file.

Why is it important?

To stay in par with other products working in cloud

Fixes: ##26137

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 Cause: Pull request #26362 updated

• Start Time: 2021-06-18T06:15:54.615+0000

• Duration: 52 min 34 sec

• Commit: 194a5a6

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Test	Results
Failed	0
Passed	6876
Skipped	24
Total	6900

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Test	Results
Failed	0
Passed	6876
Skipped	24
Total	6900