feat(doi): add DOI-based path access for commands

Support accessing repositories using DOI strings with optional subpaths
across ls, download, metadata, and file-metadata commands.

- Implement GET v3/doi/{id}/ API model and client calls
- Parse and resolve DOI paths into respective folder and files
- Extract common folder and file resolution logic to shared helpers
- Update README with example DOI-based shell commands

mdrsclient/api/__init__.py

@@ -1,9 +1,11 @@
+from mdrsclient.api.doi import DoiApi
 from mdrsclient.api.files import FilesApi
 from mdrsclient.api.folders import FoldersApi
 from mdrsclient.api.laboratories import LaboratoriesApi
 from mdrsclient.api.users import UsersApi
 
 __all__ = [
+    "DoiApi",
     "FilesApi",
     "FoldersApi",
     "LaboratoriesApi",