feat: custom user model

[atmonshi]

No description provided.

[what-the-diff]

PR Summary

• Addition of 'User' Model to Various Files
  The 'User' model has been added to both the 'b/config/zeus-bolt.php' and 'docs/getting-started/configuration.md' files. This update helps in recognizing and utilizing the 'User' model across different areas of the application.

• Refinement of Configuration File
  The Configuration.php file has been updated to return null when no model is detected within the array. This helps in preventing potential errors that may occur when there is no model found.

• Consolidation of User Model Reference in Multiple Files
  The reference model for the user has been standardized across several files (ResponseExporter.php, ManageResponses.php, Collection.php, Form.php, Response.php). The system now initially looks for the 'User' model from the BoltPlugin. If it is not found, the system reverts to using the model defined under 'auth.providers.users.model' in the configuration. This change improves code consistency and maintainability while ensuring applications function correctly when the 'User' model from BoltPlugin is unavailable.