packit: Enable downstream automation

[nforro]

This commit adds a koji_build job configured to build in a sidetag and a bodhi_update job configured to be created from a sidetag and depending on anaconda (implicitly) and anaconda_webui. A successful Koji build will trigger the bodhi_update job if a build of anaconda-webui is also in the sidetag.
In case you want to release only anaconda, you will have to tag the latest stable build of anaconda-webui into the sidetag by commenting /packit koji-tag in any anaconda-webui dist-git PR (that will satisfy the dependency but only the not-yet-released build of anaconda will be added to the update).

This PR complements rhinstaller/anaconda-webui#643.

See also: https://packit.dev/docs/fedora-releases-guide/releasing-multiple-packages

Note: the cross-package automation will not work properly until Packit configuration is updated in dist-git repos of both packages.

@KKoukiou please have a look.