Implement changelog split/merge mechanism

[cbbayburt]

Implement changelog split/merge mechanism in the changelog_test script to address changelog file conflict issues.

Original issue description:

It's a recurrent problem that will only get worse with the team growing.

Every time we backport a commit or a PR is merged there is a big chance that it will cause a file conflict in all the other open PRs. This can be quite annoying and time-consuming because we need to rebase manually and wait for the CI to run again.

What about having a dedicated changes file per person/feature and then merge everything during release time?

Example before release:

spacewalk-web.changes.lneves.clm
spacewalk-web.changes.lneves.channels
spacewalk-web.changes.julio.release_something

After Release:
merge everything into the top spacewalk-web.changes and remove the files.

This might need some work to adapt tools like TITO, but it will save up A LOT of time in the future for everyone.

Fixes https://github.com/SUSE/spacewalk/issues/9010