Add CQ and QQ Observer CLI plugins from boot steps (backport #13930)

[mergify]

Do not hard code them, also preserve user-provided plugins list

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This is an automatic backport of pull request #13930 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2d02964 has failed:

On branch mergify/bp/v4.1.x/pr-13930
Your branch is up to date with 'origin/v4.1.x'.

You are currently cherry-picking commit 2d029649a.
  (fix conflicts and 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)

Changes to be committed:
	modified:   deps/rabbit/src/rabbit_observer_cli_classic_queues.erl
	modified:   deps/rabbit/src/rabbit_observer_cli_quorum_queues.erl

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   deps/rabbit/src/rabbit_observer_cli.erl

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally