Add support for Chat Completion Service Tier

[mathewtinsley]

Describe the change

This adds support for specifying the Service Tier in a chat completion request.

Provide OpenAI documentation link

https://platform.openai.com/docs/api-reference/chat/create#chat-create-service_tier

Describe your solution

• Added service_tier to the chat completion request and response
• Added consts for the possible service_tier values, based on the documentation
  • The priority value is only available to enterprise customers and isn't in the API Reference. It is documented here: https://openai.com/api-priority-processing/

Tests

• Manually tested calling the chat completion endpoint with the service_tier to flex.
• Manually regression tested calling the endpoint without service_tier set.

Additional Context

The service_tier property is needed to use flex and priority processing:

• https://platform.openai.com/docs/guides/flex-processing
• https://openai.com/api-priority-processing/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.83%. Comparing base (d7dca83) to head (9504bf3).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   85.43%   85.83%   +0.40%     
==========================================
  Files          43       43              
  Lines        2313     2316       +3     
==========================================
+ Hits         1976     1988      +12     
+ Misses        315      305      -10     
- Partials       22       23       +1     

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[sashabaranov]

Thank you for the PR!