Use cached metadata for npm metadata endpoint at the project level

🌱 Context

At this point we should be ready to start returning a cached metadata at the project level endpoint.

last_downloaded_at field of Packages::Npm::MetadataCache will help with an observation of usage of cached metadata and will be a criteria to cleanup old caches.

⚒️ Proposal

1. Separate the instance and project level metadata endpoints.
2. Use Packages::Npm::MetadataCache with a fallback to "on the fly" generation for the project level endpoint.
3. Touch last_downloaded_at field of Packages::Npm::MetadataCache when cached metadata is returned.

Important

Hide behind a feature flag

⚠️ The case when a metadata cache entry exists in the database but a file is missing should be handled properly. This is needed to avoid errors on secondary Geo sites since NPM metadata cache files are not replicated by Geo.