Replace custom OpenAI code with official library

[waf]

When OpenAI was first released, there wasn't a good C# library, so we built the CSharpRepl + OpenAI integration on top of HttpClient. This replaces that custom implementation with the official OpenAI Nuget Package. Luckily, both the custom implementation and the official package represented the streaming text as an IAsyncEnumerable, so the transition wasn't too difficult.

[codecov]

Codecov Report

Attention: Patch coverage is 35.13514% with 24 lines in your changes missing coverage. Please review.

Project coverage is 67.2%. Comparing base (dbffe47) to head (291be09).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...pRepl.Services/Completion/OpenAICompleteService.cs	22.5%	23 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #388     +/-   ##
=======================================
- Coverage   68.2%   67.2%   -1.1%     
=======================================
  Files         98      90      -8     
  Lines       6917    6719    -198     
  Branches     929     915     -14     
=======================================
- Hits        4721    4518    -203     
- Misses      1869    1884     +15     
+ Partials     327     317     -10     

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