Consider adding a lightweight `/info_checksums` endpoint to reduce `/versions` response size

[hsbt]

Bundler and RubyGems clients currently fetch /versions to obtain gem names and their info checksums, but they do not use the version lists included in the response. A lighter-weight endpoint that omits the version numbers and returns only gem names paired with info checksums could significantly reduce response size and parsing time.

A proof-of-concept implementation for the response generation logic was proposed in rubygems/compact_index#95. That PR is being closed for now, but this issue serves as a reminder to revisit the idea when there is appetite for the full end-to-end change across compact_index, rubygems.org, and the Bundler/RubyGems client.

The work required would be:

• lib/compact_index/* from Vendor compact index into rubygems.org #6404: add response generation for the new format (as prototyped in the PR above)
• rubygems.org: add a new route and controller action serving the endpoint
• Bundler/RubyGems: update the client to prefer the new endpoint with fallback to `/versions

[paracycle]

Isn't this just the /names API endpoint with extra data? https://github.com/rubygems/compact_index?tab=readme-ov-file#names

Instead of adding a new endpoint, I think it might make sense to add metadata like checksums to the /names endpoint instead.