Add LdapClient

Closes gh-675

Author: jzheaux

core/src/main/java/org/springframework/ldap/core/ContextMapper.java

@@ -64,4 +64,6 @@ public interface ContextMapper<T> {
 	 * @throws NamingException if an error occurs.
 	 */
 	T mapFromContext(Object ctx) throws NamingException;
+
+	ContextMapper<DirContextOperations> DEFAULT = (ctx) -> (DirContextOperations) ctx;
 }