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/version.py

@@ -17,4 +17,8 @@ class VersionCommand(BaseCommand):
 
     @classmethod
     def version(cls) -> None:
-        print(f"mdrs {__version__}")
+        from mdrsclient.client import MdrsClient
+
+        # Client initialization is not strictly needed for version, but for consistency:
+        client = MdrsClient(None)
+        print(client.version())