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/__version__.py

@@ -1,5 +1,9 @@
-from importlib.metadata import version
+import importlib.metadata
+
+try:
+    __version__ = importlib.metadata.version("mdrs-client-python")
+except importlib.metadata.PackageNotFoundError:
+    __version__ = "0.0.0-dev"
 
-__version__ = version("mdrs-client-python")
 
 __all__ = ["__version__"]