Backport of 'Introduce `read_virtual_registry` custom role ability'

What does this MR do and why?

This is a backport of !231294 (merged) to the 18-10-stable-ee branch, targeting customers impacted by a regression introduced in GitLab 18.9 (#580977 (closed)).

The 18.9 change required top-level group membership to access virtual registries. This broke enterprise customers using SAML/OIDC-based access models with strict team isolation, where users are not granted top-level group membership and short-lived credentials are the only authentication mechanism — making existing workarounds (Guest access, deploy tokens, PATs) non-viable.

This MR introduces a read_virtual_registry custom role ability at the group level, allowing admins to grant virtual registry read access to minimal-access members without exposing broader group structure. This unblocks affected customers without requiring migration or relaxing their security posture.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.