Add sidetag support for koji_build

[nforro]

Tested locally, seems to work like a charm.

TODO:

• [x] Write new tests or update the old ones to cover new functionality.
• [x] Update doc-strings where appropriate.
• [ ] Update or write new documentation in packit/packit.dev.

Fixes https://github.com/packit/packit-service/issues/2377.

Merge after https://github.com/packit/packit/pull/2299.

RELEASE NOTES BEGIN

koji_build job has a new sidetag_group option that allows to perform a downstream Koji build in a sidetag. A new sidetag will be created for each configured dist_git_branch if it doesn't already exist.

RELEASE NOTES END

[softwarefactory-project-zuul[bot]]

Build failed. https://softwarefactory-project.io/zuul/t/packit-service/buildset/976481b930184ec8aac518f2321111fc

:heavy_check_mark: pre-commit SUCCESS in 2m 26s :x: packit-service-tests FAILURE in 2m 37s

[nforro]

recheck

[softwarefactory-project-zuul[bot]]

Build succeeded. https://softwarefactory-project.io/zuul/t/packit-service/buildset/f1bc48860df24ac0b4f2bd7413b2ce57

:heavy_check_mark: pre-commit SUCCESS in 2m 19s :heavy_check_mark: packit-service-tests SUCCESS in 2m 39s

[nforro]

[ ] Update or write new documentation in packit/packit.dev.

I don't think it makes much sense to document this now as practical use is quite limited until the follow-up issues are implemented.

[softwarefactory-project-zuul[bot]]

Build succeeded (gate pipeline). https://softwarefactory-project.io/zuul/t/packit-service/buildset/491efb9741bb4fd3bb1b0815981d0536

:heavy_check_mark: pre-commit SUCCESS in 11m 22s