[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/test/native/cpp/kinematics/proto/DifferentialDriveKinematicsProtoTest.cpp

@@ -0,0 +1,27 @@
+// 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 <gtest/gtest.h>
+
+#include "frc/kinematics/DifferentialDriveKinematics.h"
+#include "kinematics.pb.h"
+
+using namespace frc;
+
+namespace {
+
+using ProtoType = wpi::Protobuf<frc::DifferentialDriveKinematics>;
+
+const DifferentialDriveKinematics kExpectedData =
+    DifferentialDriveKinematics{1.74_m};
+}  // namespace
+
+TEST(DifferentialDriveKinematicsProtoTest, Roundtrip) {
+  google::protobuf::Arena arena;
+  google::protobuf::Message* proto = ProtoType::New(&arena);
+  ProtoType::Pack(proto, kExpectedData);
+
+  DifferentialDriveKinematics unpacked_data = ProtoType::Unpack(*proto);
+  EXPECT_EQ(kExpectedData.trackWidth.value(), unpacked_data.trackWidth.value());
+}