While working on the conversion of google_maps_flutter to Pigeon, I encountered an important use case for being able to access the Pigeon serialization codec: platform view creation parameters.

The plugin has a lot of structured data that it passes in Pigeon calls (map options, collections of each of the map object types, etc.), but it also passes many of those same structures in the platform view creation call. We'll want to be able to set up Pigeon data classes for all them, and then use them in both Pigeon calls and in the creation params. The engine APIs for creating platform views allow specifying a codec that's used for the passed parameter object, so if we make the Pigeon codec public, then we could make the creation parameter object a Pigeon data class instead of the current Map of string->complex maps/lists.