Add User API support and current-user endpoints

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Description

Summary

This PR adds support for the Terraform Cloud User API in the Python SDK.

It introduces a Users resource with support for:

• Reading a user by ID
• Reading the currently authenticated user
• Updating the currently authenticated user

Changes

• Implemented Users.read(user_id) using GET /api/v2/users/{user_id}
• Added read_current() and update_current() methods
• Added input validation and integrated Users into TFEClient
• Updated the User model:
  • handled nullable fields (auth_method, avatar_url, email, unconfirmed_email)
  • added TwoFactor and UserPermissions
  • aligned nullable booleans with the Go SDK
  • added UserUpdateCurrentOptions
• Parsed API responses into structured models
• Added unit tests for:
  • read(user_id)
  • invalid ID validation
  • read_current()
  • update_current()
  • nullable field handling
  • two_factor parsing
  • nullable booleans
• Added an example demonstrating read_current() and optional read(user_id)

Output

Added unit tests covering the new Users.read functionality.

Verified that the new tests pass successfully.

Confirmed that the full unit test suite runs without failures.

Run the example: