Add invitations API methods

[blairworkos]

Description

Adds methods and resources for the invitations API (part of user management)

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (fbb3dae) 86.47% compared to head (6f91ced) 86.38%.

Files	Patch %	Lines
workos/user_management.py	82.85%	6 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
- Coverage   86.47%   86.38%   -0.09%     
==========================================
  Files          29       29              
  Lines        1131     1168      +37     
==========================================
+ Hits          978     1009      +31     
- Misses        153      159       +6     

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