Implement a command to tag builds of a package into a sidetag group

[nforro]

Users need a way to get a build of a package into a Packit-managed sidetag without a rebuild (a rebuild couldn't even be possible if there is no NVR change).

Let's say that package C depends on packages A and B. Packages B and C get updated and their sync_release PRs get merged, but package A isn't updated and its last builds need to be tagged to the configured sidetag group in order for the pipeline to continue.

Implement a command, perhaps something like /packit koji-tag-latest, that will tag the latest builds (for each configured dist-git branch) of a package to a configured sidetag group.