Add sidetag support for koji_build

[nforro]

Tested locally, seems to work like a charm.

TODO:

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

Fixes #2377.

Merge after packit/packit#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]

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

✔️ pre-commit SUCCESS in 2m 26s
❌ packit-service-tests FAILURE in 2m 37s

[nforro]

recheck

[softwarefactory-project-zuul]

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

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

[lbarcziova]

LGTM 🚀

[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]

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

✔️ pre-commit SUCCESS in 11m 22s