Provide some means to generate from/toJson for compatible types for other code generators

json serializable is a powerful generator. Often, other generators will want to be able to generate to/fromJson functions for its own use.

This can include retrofit, riverpod's v3 experimental `@JsonPersist()`, and plenty more, which are effectively forced to re-implement json_serializable, which will never be as good, and will always be missing features.

I noticed this issue when i tried using `@JsonPersist()` with a type that uses generic argument factories, and it failed to generate correct code. the same was true of Slugid, which does not have a fromJson constructor, and it does not have a toJson method. with jsonSerializable, i could tell json_serializable to use `Slugid(json)` and `slugid.toString()` as is proper, but attempting to support that in riverpod would be highly unreasonable, forcing me to wrap the class to use json_serializable on, which is bad API

Often, this can involve programatically supporting generic argument factories, enums, and even classes that dont directly support serialization, which could benefit from somehow applying `@JsonKey` and Converters

I'm not sure what the best way to do this is, but it would be a benefit if we could keep all serialization code centralized _here_