Custom fields for users - re-targeted against v8

[uberbrady]

This is similar to #13715 - just re-based against the new v8 branch.

This PR has been rebased many, many, many times - so I would expect that it might have acquired some bit-rot during the process.

[what-the-diff]

PR Summary

• Introduction of Helper Methods for Custom Fields

  • Added new CustomFieldHelper.php file containing methods for handling different operations related to custom fields. This results in a streamlined data handling process.

• Refactoring of Various Controllers

  • Updated the logic of retrieving, storing, and managing custom fields in AssetsController.php, CustomFieldsetsController.php, UsersController.php and AssetsModelsController.php to further optimize the overall performance.

• Enhancements of Custom Field Management

  • Introduced changes in CustomFieldsController.php for better management of custom fields, ensuring the right context for fieldset associations.

• Code Clean-ups

  • Outdated codes and obsolete methods related to default values for custom fields were removed from AssetsController.php and AssetModel.php. This leads to a cleaner, more maintainable codebase and paves the way for new features.

• Modifications in Data Transformation Logic

  • Changed the logic of transforming custom fields in AssetsTransformer.php and UsersTransformer.php to make use of CustomFieldHelper, achieving more efficient handling of transformation processes.

• Improvement in Importing Logic

  • Cleaned up the custom field handling logic for asset imports in AssetImporter.php and filled custom fields method in Importer.php for easier understanding and potential improvements.

• Introduction of New Model, Trait, and Migration Files

  • The addition of DefaultValuesForCustomFields model, HasCustomFields trait, and a few migration files that enable new features and capabilities on managing custom fields more effectively in various models like Asset, AssetModel, and User.

• Changes in the Presentation Layer

  • Updated AssetPresenter and UserPresenter to directly use CustomFieldHelper, resulting in a more efficient data table layout creation process.

• Creation of New View Files

  • A new view file custom-fields-view.blade.php was created for better handling of how custom fields are displayed, increasing user interface consistency.

• Updates in Existing View Files

  • Changes in multiple view files like edit.blade.php and index.blade.php were made necessary for conditional rendering and improved handling of custom fields.

• Addition of New Unit Test Files

  • Introduced a new unit test file HasCustomFieldsTraitTest.php to test various aspects of custom fields for the Asset model, leading to more reliable software.