Add non-breaking TemperatureOpt field to ChatCompletionRequest that can be set to explicit zero.

[misberner]

This PR adds a TemperatureOpt *float32 field to ChatCompletionRequest, that allows distinguishing between an unset (default) temperature and explicit zero. This has been reported in several issues (e.g., #9), but this solution explicitly proposes a non-breaking change.

OpenAI doc: https://platform.openai.com/docs/api-reference/chat/create#chat-create-temperature

The new field is taken into account via custom JSON marshaling/unmarshaling functions, resorting to the legacy Temperature field (now marked as deprecated) whenever TemperatureOpt is not set. The unmarshaling logic prioritizes the legacy Temperature field whenever unambiguous to avoid incompatibilities; TemperatureOpt is only used when necessary to represent an explicit zero. A newly added GetTemperature() method allows programmatic users to get the temperature value that would be used for marshaling.

Note: There is one breaking change, which I think however can be regarded as sufficiently minor to justify ignoring it: when unmarshaling a ChatCompletionRequest with an explicit temperature setting of zero, then setting the legacy Temperature field to zero, and then re-using/re-marshaling the request, the result will be a request with an explicit zero temperature, rather than (as would previously be the case) a default temperature setting. But this seems pretty fabricated to me.

Testing is done via unit test, ensuring correct marshaling and unmarshaling behavior in various scenarious (no field set - default temperature, legacy field set, new field set, both field sets, explicit zero temperature).

Issues:

• omitempty option of request struct will generate incorrect request when parameter is 0. #9
• 0 is a valid value for temperature, top-p in the openAI API, but is not forwarded by go-openai #678

[codecov]

Codecov Report

Attention: Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.57%. Comparing base (774fc9d) to head (1fd2c83).
Report is 97 commits behind head on master.

Files with missing lines	Patch %	Lines
chat.go	92.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #983       +/-   ##
===========================================
- Coverage   98.46%   85.57%   -12.89%     
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  Files          24       43       +19     
  Lines        1364     2308      +944     
===========================================
+ Hits         1343     1975      +632     
- Misses         15      310      +295     
- Partials        6       23       +17     

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