feat(client): add placeholder methods for unimplemented API endpoints

[briansumma]

Summary

Adds placeholder methods for API endpoints that haven't been implemented yet, improving discoverability and providing a clear development roadmap.

Notes for reviewer

• The changes maintain full backward compatibility - all existing functionality
  remains unchanged, only adding new placeholder methods
• I would merge this into main before merging feature/acls and feature/transcode;
  although you may still get some merge conflicts in the client.py file that you will
  have to resolve

Changes

• Added 9 placeholder endpoint methods to PythonikClient:
  • acls() - Access Control Lists
  • auth() - Authentication
  • automations() - Workflow automations
  • notifications() - System notifications
  • settings() - Configuration settings
  • stats() - Analytics and statistics
  • transcode() - Media transcoding
  • users() - User management
  • users_notifications() - User-specific notifications

Implementation Details

• Each placeholder raises NotImplementedError with descriptive messages
• Consistent docstring format explaining the endpoint purpose
• Error messages guide users to documentation and contribution opportunities
• Maintains same method signature pattern as implemented endpoints

Benefits

• Discoverability: Users can see all planned endpoints via IDE autocomplete
• Documentation: Clear indication of what each endpoint will provide
• Consistency: Follows established patterns from existing endpoints
• Contribution: Makes it easy for contributors to identify implementation opportunities

Testing

• No breaking changes - purely additive
• Existing functionality remains unchanged
• New methods properly raise NotImplementedError as expected