java codegen: expose the extension fields

Author: mr-c

schema_salad/java_codegen.py

@@ -280,6 +280,9 @@ def begin_class(
   private LoadingOptions loadingOptions_ = new LoadingOptionsBuilder().build();
   private java.util.Map<String, Object> extensionFields_ =
       new java.util.HashMap<String, Object>();
+  public java.util.Map<String, Object> getExtensionFields() {{
+    return this.extensionFields_;
+  }}
 """.format(
                     package=self.package,
                     cls=cls,