[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/system/plant/proto/DCMotorProto.cpp

@@ -0,0 +1,35 @@
+// 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/system/plant/proto/DCMotorProto.h"
+
+#include "plant.pb.h"
+
+google::protobuf::Message* wpi::Protobuf<frc::DCMotor>::New(
+    google::protobuf::Arena* arena) {
+  return google::protobuf::Arena::CreateMessage<wpi::proto::ProtobufDCMotor>(
+      arena);
+}
+
+frc::DCMotor wpi::Protobuf<frc::DCMotor>::Unpack(
+    const google::protobuf::Message& msg) {
+  auto m = static_cast<const wpi::proto::ProtobufDCMotor*>(&msg);
+  return frc::DCMotor{
+      units::volt_t{m->nominal_voltage()},
+      units::newton_meter_t{m->stall_torque()},
+      units::ampere_t{m->stall_current()},
+      units::ampere_t{m->free_current()},
+      units::radians_per_second_t{m->free_speed()},
+  };
+}
+
+void wpi::Protobuf<frc::DCMotor>::Pack(google::protobuf::Message* msg,
+                                       const frc::DCMotor& value) {
+  auto m = static_cast<wpi::proto::ProtobufDCMotor*>(msg);
+  m->set_nominal_voltage(value.nominalVoltage.value());
+  m->set_stall_torque(value.stallTorque.value());
+  m->set_stall_current(value.stallCurrent.value());
+  m->set_free_current(value.freeCurrent.value());
+  m->set_free_speed(value.freeSpeed.value());
+}

wpimath/src/main/native/cpp/system/plant/struct/DCMotorStruct.cpp

@@ -0,0 +1,35 @@
+// 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/system/plant/struct/DCMotorStruct.h"
+
+namespace {
+constexpr size_t kNominalVoltageOff = 0;
+constexpr size_t kStallTorqueOff = kNominalVoltageOff + 8;
+constexpr size_t kStallCurrentOff = kStallTorqueOff + 8;
+constexpr size_t kFreeCurrentOff = kStallCurrentOff + 8;
+constexpr size_t kFreeSpeedOff = kFreeCurrentOff + 8;
+}  // namespace
+
+using StructType = wpi::Struct<frc::DCMotor>;
+
+frc::DCMotor StructType::Unpack(std::span<const uint8_t, kSize> data) {
+  return frc::DCMotor{
+      units::volt_t{wpi::UnpackStruct<double, kNominalVoltageOff>(data)},
+      units::newton_meter_t{wpi::UnpackStruct<double, kStallTorqueOff>(data)},
+      units::ampere_t{wpi::UnpackStruct<double, kStallCurrentOff>(data)},
+      units::ampere_t{wpi::UnpackStruct<double, kFreeCurrentOff>(data)},
+      units::radians_per_second_t{
+          wpi::UnpackStruct<double, kFreeSpeedOff>(data)},
+  };
+}
+
+void StructType::Pack(std::span<uint8_t, kSize> data,
+                      const frc::DCMotor& value) {
+  wpi::PackStruct<kNominalVoltageOff>(data, value.nominalVoltage.value());
+  wpi::PackStruct<kStallTorqueOff>(data, value.stallTorque.value());
+  wpi::PackStruct<kStallCurrentOff>(data, value.stallCurrent.value());
+  wpi::PackStruct<kFreeCurrentOff>(data, value.freeCurrent.value());
+  wpi::PackStruct<kFreeSpeedOff>(data, value.freeSpeed.value());
+}