Reintroduce fleetctl as tool

[endorama]

We recently removed fleetctl in #13440 as it's no longer used in test, and was part of systemtest.

But there is still use for it as a standalone tool to aid development and manual testing, so I'm proposing this PR that moves it in tools/

This change adds a direct dependency between tools and systemtest, but the same dependency was present, indirectly, before this PR so I don't consider it a blocker.

We may want to refactor systemtest/fleettest to split the concerns, but I'm not sure it's worth the effort at this moment.

[mergify]

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

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

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

[endorama]

I don't think we need to backport this PR.

[mergify]

This pull request is now in conflicts. Could you fix it @endorama? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b fleetctl-tool upstream/fleetctl-tool
git merge upstream/main
git push upstream fleetctl-tool