Check If toBuilder Is Implemented

Since RC1 is right around the corner, let's change the API
footprint as little as possible by using reflection to check
if a class has declared toBuilder themselves. If they have, we
can assume that that class's builder will produce that class.

Issue gh-18052

core/src/test/java/org/springframework/security/authentication/NonBuildableAuthenticationToken.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2004-present the original author or authors.
+ *
+ * Licensed 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
+ *
+ *      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.springframework.security.authentication;
+
+public class NonBuildableAuthenticationToken extends TestingAuthenticationToken {
+
+	public NonBuildableAuthenticationToken(String user, String password, String... authorities) {
+		super(user, password, authorities);
+	}
+
+}