[BREAKING_CHANGES] Make temperature nullable so that it can be set to 0

[gburt]

For the Chat and Completions APIs make the Temperature field a *float32 so we can distinguish between 0 and null. Users often want to set the temperature to 0, but right now that's not possible because then the temperature field is omitempty'd from the JSON request object.

Other numeric parameters already default to 0 or have 0 as a nonsensical value, so this is only needed, AFAICT, for these two params.

This is an API break, but I think it's one worth making. We've been unintentionally running queries at the default temp (1) when we meant to be running it at 0.

Fixes #9

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.58%. Comparing base (774fc9d) to head (1192948).
Report is 78 commits behind head on master.

Files with missing lines	Patch %	Lines
common.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #922      +/-   ##
==========================================
+ Coverage   98.46%   98.58%   +0.12%     
==========================================
  Files          24       27       +3     
  Lines        1364     1765     +401     
==========================================
+ Hits         1343     1740     +397     
- Misses         15       18       +3     
- Partials        6        7       +1     

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