Unique constraint for Advisory packages

[pavelpicka]

Describe the bug
When sync pulp doesn't check unique constrain advisory package so duplicate could appear in DB.

To Reproduce
right now to sync Alma linux repositories (BaseOS, AppStream and PowerTools)

Expected behavior
Deduplication at sync time.

We need to imporove advisory package unique constrain.

[ggainey]

This fixes at a higher level the problem being addressed in #2648 . It is def useful to "fix" data like this as it comes in, we want to keep the current fix ( #2727 ) since it doesn't require a migration or a data fix and will fix existing users who have already sync'd, exported, and are attempting to import the suboptimal repositories.

After discussion around the implications of "mirror", we've realized that this doesn't break mirror - we de-dup the data in "our" database, but the published-metadata artifact is "whatever we sync'd from the remote". Import/export is unaffected because we do not export Publications or Distributions.