Support Project ID

[pkosiec]

Describe the change
Please provide a clear and concise description of the changes you're proposing. Explain what problem it solves or what feature it adds.

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/organizations-and-projects-optional

Describe your solution
I've added an optional header, same as the Organization ID.

As @massenz mentioned here, while the Project ID it is optional, it would be great to match the official OpenAI client and have a double check for the project scope.

Tests
I've tested the changes manually.

Resolves #757

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.51%. Comparing base (774fc9d) to head (a666b26).
Report is 18 commits behind head on master.

Files	Patch %	Lines
client.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
+ Coverage   98.46%   98.51%   +0.05%     
==========================================
  Files          24       24              
  Lines        1364     1143     -221     
==========================================
- Hits         1343     1126     -217     
+ Misses         15       10       -5     
- Partials        6        7       +1     

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[massenz]

Thanks for the patch!
The change looks good to me ✅