✨ Add support for the whenever package

Added a full set of annotatted classes to enable `whenever` types to be
used with Pydantic.

The following annotated classes were added
* Core types
  * InstantAnnotation,
  * LocalDateTimeAnnotation,
  * ZonedDateTimeAnnotation,
* Advanced types
  * OffsetDateTimeAnnotation,
  * SystemDateTimeAnnotation,
* Deltas
  * DateDeltaAnnotation,
  * DateTimeDeltaAnnotation,
  * TimeDeltaAnnotation,

Also, a full set of unit tests was added.

Author: dusktreader