Implement GetTimeZoneTransition method for ZonedDateTime

[nekevss]

Related to ongoing work for #13.

The method GetTimeZoneTransition needs to be implemented for ZonedDateTime.

This may involve extending the TzProvider trait and implementing that extension on FsTzdbProvider.

Initial thought on how the implementation may look would be to add the method like below:

trait TzProvider {
  // ...
  // Existing trait methods
  
  fn get_timezone_transition(&self, direction: TransitionDirection) -> EpochNanoseconds;
}