feat: Implement DateTimeZone

[chrisimcevoy]

Implements the vast majority of DateTimeZone functionality.

Not immediately sure what to do with the DateTimeZone.GetZoneIntervals() overload which accepts ZoneEqualityComparer.Options. I'll come back to that.

Relates to:

• Provide an example in the quick start section of README.md #232

[codecov]

Codecov Report

Attention: Patch coverage is 98.99598% with 5 lines in your changes missing coverage. Please review.

Project coverage is 96.22%. Comparing base (090452a) to head (553504f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pyoda_time/_date_time_zone.py	93.33%	3 Missing ⚠️
pyoda_time/_offset_date_time.py	95.83%	1 Missing ⚠️
pyoda_time/_zoned_date_time.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   95.99%   96.22%   +0.22%     
==========================================
  Files         259      260       +1     
  Lines       19606    20081     +475     
==========================================
+ Hits        18821    19323     +502     
+ Misses        785      758      -27     

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