[cloud_firestore_odm_generator] Support Firestore Timestamps

[FXschwartz]

Currently when running flutter pub run build_runner build --delete-conflicting-outputs against a model that contains a timestamp throws an error.

Could not generate `fromJson` code for `pressedOn`.
To support the type `Timestamp` you can:
* Use `JsonConverter`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
* Use `JsonKey` fields `fromJson` and `toJson`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html
package:buttonSimple/models/button_event.dart:20:19

Looks like this is on json_serializable to implement but it might help to have an issue here to track for people running into this and assuming it's a bug.

Currently, there is an open enhancement in the json_serializable repo

[inamit]

Is there a workaround for this issue for now? I have tried making the field a DateTime and implementing a JsonConverter, but flutter pub run build_runner build --delete-conflicting-outputs doesn't generate a where or orderby functions for this field.

[FXschwartz]

I haven't found a fully working workaround yet, all of my projects have DateTime fields so I, unfortunately, can't use ODM until this gets implemented.