Design Doc: Extensible Enums in Federated Plugins

### Document Link

https://flutter.dev/go/extensible-enums-plugins
Direct link: https://docs.google.com/document/d/1wHQjtse99mNJpJk-ctv7zrKSq4jgzpLRfqkaLzEsh_E/edit?tab=t.0

### What problem are you solving?

To establish a clear, maintainable architectural pattern for exposing platform-specific constants (e.g., `ImageFormatGroup` in camera plugins) in Flutter federated plugins. This system must allow value sets to safely grow as underlying native APIs evolve (iOS, Android, Web, etc), without introducing breaking changes or forcing major version bumps for downstream developers using exhaustive switch statements.

Start by reading the linked Flutter design document and the stated problem around extensible platform-specific constants in federated plugins. Determine the architectural pattern needed across iOS, Android, Web, and other platforms; done means the design is clearly documented and establishes how value sets can grow without breaking downstream exhaustive switches.

Written by the indexing model from the issue text.