Support limit pushdown for OneOfExec (#94)

Author: xudong963

src/rewrite/exploitation.rs

@@ -450,6 +450,10 @@ impl ExecutionPlan for OneOfExec {
         )?))
     }
 
+    fn supports_limit_pushdown(&self) -> bool {
+        true
+    }
+
     fn execute(
         &self,
         partition: usize,