🔧 refactor(documentloaders.py): extract build_directory_loader_fields() function to improve code readability and maintainability

🔧 refactor(documentloaders.py): remove redundant code for adding fields in DocumentLoaderFrontNode class
The code in the DocumentLoaderFrontNode class has been refactored to extract the logic for building directory loader fields into a separate function called build_directory_loader_fields(). This improves code readability and maintainability by separating concerns and reducing duplication. The redundant code for adding fields in the DocumentLoaderFrontNode class has also been removed.