Design Additional Root Registry APIs

[theory]

In addition to the APIs designed for basic root registry operation in #2 (more or less equivalent to pgxn-manager, design additional APIs required for the operation of the Web site, clients, and other services (more or less equivalent to pgxn-api. Relevant APIs:

• [ ] Extension discovery
• [ ] Metadata display
• [ ] Full text search
• [ ] Release profiles
• [ ] Extension profiles
• [ ] User profiles
• [ ] Classifications
• [ ] Feeds (Atom & JSON Feed)
• [ ] Source browsing
• [ ] Download API
• [ ] Downstream Packages
• [ ] Stats & Reports
• [ ] Extension Documentation