[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/proto/DifferentialDriveWheelSpeedsProto.cpp

@@ -0,0 +1,32 @@
+// 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/proto/DifferentialDriveWheelSpeedsProto.h"
+
+#include "kinematics.pb.h"
+
+google::protobuf::Message* wpi::Protobuf<
+    frc::DifferentialDriveWheelSpeeds>::New(google::protobuf::Arena* arena) {
+  return google::protobuf::Arena::CreateMessage<
+      wpi::proto::ProtobufDifferentialDriveWheelSpeeds>(arena);
+}
+
+frc::DifferentialDriveWheelSpeeds
+wpi::Protobuf<frc::DifferentialDriveWheelSpeeds>::Unpack(
+    const google::protobuf::Message& msg) {
+  auto m = static_cast<const wpi::proto::ProtobufDifferentialDriveWheelSpeeds*>(
+      &msg);
+  return frc::DifferentialDriveWheelSpeeds{
+      units::meters_per_second_t{m->left()},
+      units::meters_per_second_t{m->right()},
+  };
+}
+
+void wpi::Protobuf<frc::DifferentialDriveWheelSpeeds>::Pack(
+    google::protobuf::Message* msg,
+    const frc::DifferentialDriveWheelSpeeds& value) {
+  auto m = static_cast<wpi::proto::ProtobufDifferentialDriveWheelSpeeds*>(msg);
+  m->set_left(value.left.value());
+  m->set_right(value.right.value());
+}