[7.16](backport #28461) Add diagnostics collect command to elastic-agent.

[mergify]

This is an automatic backport of pull request #28461 done by Mergify.
Cherry-pick of 99ebf3e has failed:

On branch mergify/bp/7.16/pr-28461
Your branch is up to date with 'origin/7.16'.

You are currently cherry-picking commit 99ebf3e437.
  (all conflicts fixed: run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

nothing to commit, working tree clean

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[botelastic]

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[elasticmachine]

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• Start Time: 2021-11-10T16:24:50.218+0000

• Duration: 15 min 35 sec

• Commit: 60cc702

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[michel-laterman]

not sure why this was created.