Add Pulpcore S3 storage metapackage

[jakduch]

Summary

Add python3.12-pulpcore+s3 and the stable pulpcore-storage-s3 capability for installers and images. It pulls in Pulpcore and the django-storages boto3 extra while leaving filesystem storage as the default.

Dependency

This depends on #3186, which restores django-storages for EL9 and EL10. It is the current successor to #1760.

Consumers: theforeman/puppet-pulpcore#418 and theforeman/pulp-oci-images#117.

Verification

• EL9 and EL10 comps XML validates with xmllint.
• The Pulpcore RPM spec expands with rpmspec.
• git diff --check passes.

AI assistance disclosure: OpenAI Codex (GPT-5.6 Sol High).

[jakduch]

Both EL9 and EL10 scratch builds pass. The remaining repoclosure failure is the expected dependency ordering: python3.12-django-storages+boto3 from #3186 is not published in the nightly staging repository yet. #3186 is green; after it is merged and published, this check can be rerun without changing this commit.