refactor: extract MdrsClient service layer for library portability

To improve the tool's portability as a Python library, the core logic
has been decoupled from the CLI interface. This allows developers to
programmatically interact with MDRS without relying on CLI-specific
argument parsing or local file-based caches.

- Introduce `MdrsClient` service layer to handle core operations.
- Abstract authentication state using `CacheInterface` and `InMemoryCache`.
- Migrate all CLI commands to utilize `MdrsClient` for execution.
- Separate `Doi` data model from API responses and move to `models/doi.py`.
- Update `README.md` to include Python API usage examples.
- Bump package version to 1.3.17.

mdrsclient/commands/metadata.py

@@ -23,8 +23,8 @@ class MetadataCommand(BaseCommand):
     @classmethod
     def metadata(cls, remote_path: str, password: str | None) -> None:
         remote = remote_path.split(":", 1)[0] if ":" in remote_path else ""
-        connection = cls._create_connection(remote)
-        folder, laboratory = cls._resolve_folder(connection, remote_path, password)
-        folder_api = FoldersApi(connection)
-        metadata = folder_api.metadata(folder.id)
+        from mdrsclient.client import MdrsClient
+
+        client = MdrsClient.from_remote(remote)
+        metadata = client.metadata(remote_path, password)
         print(json.dumps(metadata, ensure_ascii=False))