[wpiutil] Add serializable marker interfaces (#6060)

wpiutil/src/main/java/edu/wpi/first/util/WPISerializable.java

@@ -0,0 +1,8 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.util;
+
+/** Marker interface to indicate a class is serializable using WPI serialization methods. */
+public interface WPISerializable<T> {}