[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/cpp/kinematics/struct/DifferentialDriveKinematicsStruct.cpp

@@ -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.
+
+#include "frc/kinematics/struct/DifferentialDriveKinematicsStruct.h"
+
+namespace {
+constexpr size_t kTrackWidthOff = 0;
+}  // namespace
+
+using StructType = wpi::Struct<frc::DifferentialDriveKinematics>;
+
+frc::DifferentialDriveKinematics StructType::Unpack(
+    std::span<const uint8_t, kSize> data) {
+  return frc::DifferentialDriveKinematics{
+      units::meter_t{wpi::UnpackStruct<double, kTrackWidthOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::DifferentialDriveKinematics& value) {
+  wpi::PackStruct<kTrackWidthOff>(data, value.trackWidth.value());
+}