Make JsonKey's defaultValue a Function? type

[erf]

I would like to supply a DateTime.now() as a defaultValue to JsonKey, but as it's not a const it's not possible to add it, however if it was of type Function? i could pass it a function which would return a value in the same way as toJson and fromJson, e.g. static DateTime newDate() => DateTime.now(); This would be a breaking change so not sure if it's feasible, but it would make the API more flexible perhaps?

Dart SDK version: 2.17.6 (stable) (Tue Jul 12 12:54:37 2022 +0200) on "macos_arm64"