[wpiutil] Add serializable marker interfaces

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> {}

wpiutil/src/main/java/edu/wpi/first/util/protobuf/ProtobufSerializable.java

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+// 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.protobuf;
+
+import edu.wpi.first.util.WPISerializable;
+
+/**
+ * Marker interface to indicate a class is serializable using Protobuf serialization.
+ *
+ * <p>While this cannot be enforced by the interface, any class implementing this interface should
+ * provide a public final static `proto` member variable.
+ */
+public interface ProtobufSerializable<T> extends WPISerializable<T> {}

wpiutil/src/main/java/edu/wpi/first/util/struct/StructSerializable.java

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+// 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.struct;
+
+import edu.wpi.first.util.WPISerializable;
+
+/**
+ * Marker interface to indicate a class is serializable using Struct serialization.
+ *
+ * <p>While this cannot be enforced by the interface, any class implementing this interface should
+ * provide a public final static `struct` member variable.
+ */
+public interface StructSerializable<T> extends WPISerializable<T> {}