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/connection.py

@@ -9,7 +9,7 @@ from requests_toolbelt.multipart.encoder import MultipartEncoder
 from typing_extensions import Unpack
 
 from mdrsclient.__version__ import __version__
-from mdrsclient.cache import CacheFile
+from mdrsclient.cache import CacheFile, CacheInterface
 from mdrsclient.exceptions import MissingConfigurationException
 from mdrsclient.models import Laboratories, Token, User
 
@@ -39,14 +39,14 @@ class MDRSConnection:
     url: str
     session: Session
     lock: threading.Lock
-    __cache: CacheFile
+    __cache: CacheInterface
 
-    def __init__(self, remote: str, url: str) -> None:
+    def __init__(self, remote: str, url: str, cache: CacheInterface | None = None) -> None:
         super().__init__()
         self.url = url
         self.session = Session()
         self.lock = threading.Lock()
-        self.__cache = CacheFile(remote)
+        self.__cache = cache if cache is not None else CacheFile(remote)
         self.__prepare_headers()
 
     def get(self, url: str, **kwargs: Unpack[_KwArgsMDRSConnectionGet]) -> Response: