feat(cdm): Add fallback to Widevine common cert for L1 devices

- Use default Widevine common privacy certificate when no service certificate is provided for L1 devices
- Add get_widevine_service_certificate method to EXAMPLE service for config-based certificates
- Improve certificate handling with more descriptive return messages

unshackle/services/EXAMPLE/__init__.py

@@ -282,6 +282,10 @@ class EXAMPLE(Service):
 
         return chapters
 
+    def get_widevine_service_certificate(self, **_: any) -> str:
+        """Return the Widevine service certificate from config, if available."""
+        return self.config.get("certificate")
+
     def get_playready_license(self, *, challenge: bytes, title: Title_T, track: AnyTrack) -> Optional[bytes]:
         """Retrieve a PlayReady license for a given track."""