feat: Add CustomSourceOptions to SavedDatasetStorage (#2958)

Add CustomSourceOptions to SavedDatasetStorage

I am implementing a custom OfflineStore and found that I couldn't use CustomSourceOptions in
SavedDatasetStorage: currently you'd have to try to fit your custom options into one of the
existing implemented Options structs

Signed-off-by: William Horton <william.horton@grandrounds.com>

protos/feast/core/SavedDataset.proto

@@ -58,6 +58,7 @@ message SavedDatasetStorage {
     DataSource.SnowflakeOptions snowflake_storage = 7;
     DataSource.TrinoOptions trino_storage = 8;
     DataSource.SparkOptions spark_storage = 9;
+    DataSource.CustomSourceOptions custom_storage = 10;
   }
 }