Add new including_default_value_without_presence_fields to Python JSON serializer.

This flag has consistent behavior between proto2 and proto3 optionals (by not including either one), unlike including_default_value_fields which does include proto2 optional but excludes proto3 optionals.

including_default_value_fields is now deprecated and will be removed in an upcoming release.

PiperOrigin-RevId: 603156447

Author: protobuf-github-bot