Add validity bounds to the TzOffset API and support for iteration over TzOffsets

[gharveymn]

Motivation: Certain event driven programs require scheduling based on changes to the offset which is active for a given timezone.

The changes here essentially turn TzOffset into a cursor over the offsets for a given timezone.

[djc]

I think this is too invasive for a niche use case.

Is there API we could offer that would allow you to implement this outside this crate?

[robertbastian]

I think this is a useful API, I was looking for something like it as well. Getting the next transition is a common TZDB API:

• https://docs.rs/jiff/latest/jiff/tz/struct.TimeZone.html#method.following
• https://www.joda.org/joda-time/apidocs/org/joda/time/DateTimeZone.html#nextTransition-long-