Add identifier strategy for ClientRegistration

Fixes gh-4561

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/ClientRegistrationIdentifierStrategy.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2012-2017 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
+ *
+ *      http://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.oauth2.client.registration;
+
+/**
+ * A strategy for obtaining a unique identifier for a {@link ClientRegistration}.
+ *
+ * @author Joe Grandja
+ * @since 5.0
+ * @see ClientRegistration
+ */
+public interface ClientRegistrationIdentifierStrategy<T> {
+
+	T getIdentifier(ClientRegistration clientRegistration);
+
+}