add to_primitive for OffsetDateTime

[PacificBird]

When working with my legacy database at work while trying to have a sensible internal API for dealing with times I need to downcast OffsetDateTimes to PrimitiveDateTimes constantly. I was very surprised to find this isn't built-in since it is simple but needlessly verbose to achieve with the existing API. I cannot express how much nicer my code will become with this method available. Being able to go from wrapping it in a free function to a method call would be lovely.

[jhpratt]

For the love of god, search before opening a PR! You're wasting everyone's time by not doing so.

#369
#390
#395
#454
#458
#553
#658
#673
#705

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.0%. Comparing base (ac010e8) to head (5aca771).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
time/src/offset_date_time.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #771     +/-   ##
=======================================
- Coverage   94.2%   94.0%   -0.2%     
=======================================
  Files         96      97      +1     
  Lines      10732   11139    +407     
=======================================
+ Hits       10107   10473    +366     
- Misses       625     666     +41     

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