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/models/__init__.py

@@ -1,3 +1,4 @@
+from mdrsclient.models.doi import Doi
 from mdrsclient.models.error import DRFStandardizedErrors
 from mdrsclient.models.file import File
 from mdrsclient.models.folder import Folder, FolderAccessLevel, FolderSimple
@@ -6,6 +7,7 @@ from mdrsclient.models.user import Token, User
 
 __all__ = [
     "DRFStandardizedErrors",
+    "Doi",
     "File",
     "Folder",
     "FolderAccessLevel",