Add API support for file checksums in Generic Package Repository

Problem

User Need: Users need a way to verify file integrity when downloading files from the Generic Package Repository by accessing SHA256 checksums.

Current Limitation: The Generic Package Repository doesn't currently provide APIs or UI elements to access file checksums for verification purposes.

Background

This issue originated from customer feedback on the GitLab documentation:

• Documentation page: https://docs.gitlab.com/user/packages/generic_packages/?tab=With%20GitLab%20CI%2FCD
• User feedback: "Could not find what I needed" - specifically looking for "URL for sha256 associated value to each file"
• Stage/Group: Package, Package Registry

Proposed Solution

Return file checksums in response headers when downloading files from the Generic Package Repository.

Implementation considerations:

• Include SHA256 checksum in HTTP response headers (e.g., X-Checksum-SHA256 or similar)
• Consider adding checksum information to API responses for file listings
• Update documentation to explain how users can access and verify checksums

Acceptance Criteria

• File downloads from Generic Package Repository include checksum in response headers
  • Users can programmatically access file checksums for verification
  • Documentation is updated to explain checksum verification process
  • Consider API endpoints that list files with their checksums

Labels

~"Category:Generic Package Repository" ~"type::feature" ~"group::package registry"

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