fix: adds underscore_reference_keys support to interfaces

In a previous Pull Request (Gusto#248) there was added support for underscoring the keys and while this covered the `Object` class, it didn't also cover the `Interface` module. This commit adds the same `underscore_reference_keys` method to the `Interface` module.

lib/apollo-federation/interface.rb

@@ -35,6 +35,18 @@ def key(fields:, camelize: true)
           ],
         )
       end
+
+      def underscore_reference_keys(value = nil)
+        if value.nil?
+          if @underscore_reference_keys.nil?
+            find_inherited_value(:underscore_reference_keys, false)
+          else
+            @underscore_reference_keys
+          end
+        else
+          @underscore_reference_keys = value
+        end
+      end
     end
   end
 end