feat: baseline `TzdbDateTimeZoneSource` functionality

[chrisimcevoy]

Continuing with the theme of time zone functionality for the 0.8.0 release, this is an almost complete port of TzdbDateTimeZoneSource.

The only functionality that is missing is the one public method which depends on TimeZoneInfo, namely GetSystemDefaultId. I will follow up on that another time.

The tests here prove that we can read the Tzdb.nzd files which are produced by NodaTime.TzdbCompiler.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.74503% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 95.42%. Comparing base (8e3326c) to head (f994262).

Files	Patch %	Lines
...yoda_time/time_zones/_caching_zone_interval_map.py	92.68%	6 Missing :warning:
...oda_time/time_zones/_tzdb_date_time_zone_source.py	97.10%	4 Missing :warning:
pyoda_time/time_zones/_fixed_date_time_zone.py	88.46%	3 Missing :warning:
pyoda_time/time_zones/_cached_date_time_zone.py	93.75%	2 Missing :warning:
pyoda_time/time_zones/io/_tzdb_stream_data.py	93.93%	2 Missing :warning:
pyoda_time/_date_time_zone.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #146 +/- ## ========================================== + Coverage 95.28% 95.42% +0.14% ========================================== Files 227 231 +4 Lines 17640 18182 +542 ========================================== + Hits 16808 17350 +542 Misses 832 832 ```

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