Add Users resource with read and current-user endpoints

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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.

Run the Examples

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Closing this PR due to linting and formatting issues. Will raise a fresh PR with a clean branch and fixes applied.