Consider using package for json serialization

[kenzieschmoll]

Throughout DevTools, we manually implement serialization methods like .parse constructors or toJson methods / json getters.

We should consider using a package like https://pub.dev/packages/json_serializable to simplify and unify how we serialize dart objects. When choosing a package, we'll need to make sure everything will still work well in g3.

[jacob314]

To work with g3 we'd probably want to use built_value.