Enable SSRF for Generic Package Registry

What does this MR do and why?

Implements the ideas from !184626 (merged) and applies them to the generic package registry. Other package formats will be covered in subsequent MRs.

More details in https://gitlab.com/gitlab-org/gitlab/-/issues/520294

References

#520294

Screenshots or screen recordings

NA

How to set up and validate locally

See https://gitlab.com/gitlab-org/gitlab/-/issues/520294#note_2583110420 (internal)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #520294