Add beat/process config to elastic agent diagnostics collect

[michel-laterman]

What does this PR do?

Add the configs that are output from elastic-agent inspect output to the
bundle generated by elastic-agent diagnostics collect. Files are named
as ProcID_RouteKey.

Why is it important?

Include the config that's sent from the agent to underlying processes in the diagnostic bundles to allow us to more easily debug config errors.

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.

How to test this PR locally

Start an agent.
Run elastic-agent diagnostics collect once the agent is healthy
Inspect archive's config files.

Related issues

• Closes elastic-agent diagnostics: filebeat and metricbeat configuration #29898

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Start Time: 2022-01-20T16:47:33.573+0000

• Duration: 96 min 38 sec

• Commit: b82e29e

Test stats 🧪

Test	Results
Failed	0
Passed	7176
Skipped	16
Total	7192

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[jlind23]

@faec may help in some of your SDHs

[blakerouse]

Looks good.

[ph]

Changes LGTM, I am been trying to test locally but other stuff is block my build.