[FLINK-19836] Add SerializableSupplier in flink-core

This existed in tests before but I'm planning to use this in
SimpleVersionedSerializerTypeSerializerProxy and it fits with the other
special purpose Suppliers.

flink-core/src/main/java/org/apache/flink/util/function/SerializableSupplier.java

@@ -0,0 +1,33 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http:https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.util.function;
+
+import org.apache.flink.annotation.Internal;
+
+import java.io.Serializable;
+import java.util.function.Supplier;
+
+/**
+ * A serializable {@link Supplier}.
+ *
+ * @param <T> the type of results supplied by this supplier
+ */
+@Internal
+@FunctionalInterface
+public interface SerializableSupplier<T> extends Supplier<T>, Serializable {}

flink-core/src/test/java/org/apache/flink/api/java/ClosureCleanerTest.java

@@ -22,6 +22,7 @@
 import org.apache.flink.api.common.InvalidProgramException;
 import org.apache.flink.api.common.functions.MapFunction;
 import org.apache.flink.api.java.tuple.Tuple1;
+import org.apache.flink.util.function.SerializableSupplier;
 
 import org.junit.Assert;
 import org.junit.Test;
@@ -30,7 +31,6 @@
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
 import java.io.Serializable;
-import java.util.function.Supplier;
 
 /**
  * Tests for {@link ClosureCleaner}.
@@ -454,11 +454,6 @@ public MapFunction<Integer, Integer> getMap() {
  }
 }
 
-@FunctionalInterface
-interface SerializableSupplier<T> extends Supplier<T>, Serializable {
-
-}
-
 class NestedSelfReferencing implements Serializable {
 
  private final SerializableSupplier<NestedSelfReferencing> cycle;