add wrapper function to obtain credentials (esp. testing)

src/delta_barth/management.py

@@ -25,6 +25,11 @@ def set_credentials(
     )
 
 
+def get_credentials() -> str:  # pragma: no cover
+    creds = SESSION.creds
+    return creds.model_dump_json()
+
+
 def set_base_url(
     base_url: str,
 ) -> None:  # pragma: no cover