feat: admin: add invite client and tests

[MattDavisRV]

Describe the change
Adds functionality to manage user invites via the Admin API Endpoint. Includes all endpoints available within the Invite space.

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/invite

Describe your solution
Describe how your changes address the problem or how they add the feature. This should include a brief description of your approach and any new libraries or dependencies you're using.

Tests
Mocks API calls for each function, validating required fields and returns a response. Followed existing testing patterns in the repo.

Additional context
I plan to add more admin functionality, the code has written, but it was requested that I submit smaller PRs for easier review.

Issue: #XXXX

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.20%. Comparing base (5d7a276) to head (215ba63).

Files with missing lines	Patch %	Lines
admin_invite.go	82.60%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
- Coverage   99.59%   99.20%   -0.39%     
==========================================
  Files          34       35       +1     
  Lines        2209     1887     -322     
==========================================
- Hits         2200     1872     -328     
- Misses          6        8       +2     
- Partials        3        7       +4     

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