feat: [Python] add RegisterFiles so gcs files can be used with genai.

PiperOrigin-RevId: 839396637

Author: MarkDaoust

src/main/java/com/google/genai/types/FileSource.java

@@ -34,6 +34,8 @@ public enum Known {
 
     GENERATED,
 
+    REGISTERED,
+
     FILE_SOURCE_UNSPECIFIED
   }