Adding new moderation model constants

[Mhjacobs]

Describe the change
This change enables go-openai to use the omni moderation models that are now available to us from openai.
Provide OpenAI documentation link
Provide a relevant API doc from https://platform.openai.com/docs/models/moderation

Describe your solution
These changes define the constants for the new moderation models as specified by openAI, and then puts them into the map of allowed models for the moderation endpoint. I have tested these strings directly against the moderation api to confirm that these requests return a non-error response and specify the same model that is passed in.
Tests
Briefly describe how you have tested these changes. If possible — please add integration tests.
Unit tests, with the addition of the new models.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (774fc9d) to head (65da6fc).
Report is 62 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #875      +/-   ##
==========================================
+ Coverage   98.46%   99.04%   +0.58%     
==========================================
  Files          24       26       +2     
  Lines        1364     1461      +97     
==========================================
+ Hits         1343     1447     +104     
+ Misses         15        8       -7     
  Partials        6        6              

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[sashabaranov]

Thank you!