[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/DifferentialDriveWheelSpeedsStruct.cpp

@@ -0,0 +1,26 @@
+// 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/DifferentialDriveWheelSpeedsStruct.h"
+
+namespace {
+constexpr size_t kLeftOff = 0;
+constexpr size_t kRightOff = kLeftOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::DifferentialDriveWheelSpeeds>;
+
+frc::DifferentialDriveWheelSpeeds StructType::Unpack(
+    std::span<const uint8_t, kSize> data) {
+  return frc::DifferentialDriveWheelSpeeds{
+      units::meters_per_second_t{wpi::UnpackStruct<double, kLeftOff>(data)},
+      units::meters_per_second_t{wpi::UnpackStruct<double, kRightOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::DifferentialDriveWheelSpeeds& value) {
+  wpi::PackStruct<kLeftOff>(data, value.left.value());
+  wpi::PackStruct<kRightOff>(data, value.right.value());
+}