Support Rust librespot credential format in stored_file()

Adds fallback logic to load credentials from Rust librespot's OAuth flow.
The stored_file() method now handles both Python and Rust credential formats.

Related to #277

README.md

@@ -91,6 +91,18 @@ session = Session.Builder() \
     .create()
 ```
 
+### Use Stored Credentials for Login
+
+```python
+from librespot.core import Session
+
+# Supports both Python and Rust librespot credential formats
+
+session = Session.Builder() \
+    .stored_file("/path/to/credentials.json") \
+    .create()
+```
+
 ### Get Spotify's OAuth token
 
 ```python