beater: watch output config changes

[axw]

Motivation/summary

Watch libbeat output config changes, and restart the beater with updated config. This enables us to propagate the Elasticsearch output config to tail-based sampling when running under Fleet.

Checklist

- [ ] Update CHANGELOG.asciidoc (non user facing change)
- [ ] Documentation has been updated

How to test these changes

Can't be manually tested yet, needs UI for configuring tail-sampling policies.

Related issues

#5490

[apmmachine]

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Build stats

• Start Time: 2021-10-05T10:06:46.727+0000

• Duration: 46 min 6 sec

• Commit: 99d4b02

Test stats 🧪

Test	Results
Failed	0
Passed	6145
Skipped	14
Total	6159

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[simitt]

LGTM; thanks for adding the comments.

[mergify]

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

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./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.