[wpimath] Add protobuf/struct for trivial types (#5935)

This implements de/serialization for the types that aren't templated (SwerveDriveKinematics) in C++ or where there is no trivial way to go round-trip (Splines) for the messages.

wpimath/src/main/native/include/frc/kinematics/struct/DifferentialDriveKinematicsStruct.h

@@ -0,0 +1,23 @@
+// 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.
+
+#pragma once
+
+#include <wpi/SymbolExports.h>
+#include <wpi/struct/Struct.h>
+
+#include "frc/kinematics/DifferentialDriveKinematics.h"
+
+template <>
+struct WPILIB_DLLEXPORT wpi::Struct<frc::DifferentialDriveKinematics> {
+  static constexpr std::string_view kTypeString =
+      "struct:DifferentialDriveKinematics";
+  static constexpr size_t kSize = 8;
+  static constexpr std::string_view kSchema = "double track_width";
+
+  static frc::DifferentialDriveKinematics Unpack(
+      std::span<const uint8_t, kSize> data);
+  static void Pack(std::span<uint8_t, kSize> data,
+                   const frc::DifferentialDriveKinematics& value);
+};