Add GET /internal/files. (#4295)

* Create internal route table.

* List files.

* Add GET /internal/files.

Retrieves list of files in models, output, and user directories.

* Refactor file names.

* Use typing_extensions for Python 3.8

* Fix tests.

* Remove print statements.

* Update README.

* Add output and user to valid directory test.

* Add missing type hints.

api_server/services/file_service.py

@@ -0,0 +1,13 @@
+from typing import Dict, List, Optional
+from api_server.utils.file_operations import FileSystemOperations, FileSystemItem
+
+class FileService:
+    def __init__(self, allowed_directories: Dict[str, str], file_system_ops: Optional[FileSystemOperations] = None):
+        self.allowed_directories: Dict[str, str] = allowed_directories
+        self.file_system_ops: FileSystemOperations = file_system_ops or FileSystemOperations()
+
+    def list_files(self, directory_key: str) -> List[FileSystemItem]:
+        if directory_key not in self.allowed_directories:
+            raise ValueError("Invalid directory key")
+        directory_path: str = self.allowed_directories[directory_key]
+        return self.file_system_ops.walk_directory(directory_path)