[catalog-server] Manifests have a max size of 1MB

[justinfagnani]

Firestore has a limitation for the maximum field size of 1MB. Some manifests in the wild already exceed this size.

Options:

1. Always use unpkg or similar CDN for manifests. This would add latency and network ingress for every page request, but unpkg is cached by Cloudflare and very fast.
2. Store manifests in Cloud Storage. Cheap, fast, the preferred architecture for GCP. Requires making a blob storage interface for the Catalog class to use.
3. Destructure the manifest into actual Firestore fields. JSON maps to Firestore's document model very well, and the manifest format is relatively shallow - each package contains a flat list of modules. Modules would exceed 1MB exceedingly rarely. This would allow for some more structured searching of manifest data, but we don't have use-cases for that yet.
4. Compress the mainfest. Easy to do with current Firestore schema. We can serve the gzipped data directly, saving CPU.