Add a CI for packaging use-cases

[praiskup]

It would be nice to submit copr build (both scl and non-scl) before each commit is merged (at least).

[praiskup]

The SCL use-case is probably no longer needed, but having a real "testing PostgreSQL RPM build" prior to merging a PR is totally needed.

[praiskup]

There's the Packit + Testing Farm out there today to use. With that we can pretty easily have pre-merge tests like "is postgresql-setup --upgrade still working?"

[fila43]

@praiskup thank you for this point