[wpimath] Move struct/proto classes to separate files (#5918)

Also add unit tests.

wpimath/src/test/native/cpp/geometry/proto/Pose3dProtoTest.cpp

@@ -0,0 +1,29 @@
+// 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/geometry/Pose3d.h"
+#include "geometry3d.pb.h"
+
+using namespace frc;
+
+namespace {
+
+using ProtoType = wpi::Protobuf<frc::Pose3d>;
+
+const Pose3d kExpectedData =
+    Pose3d{Translation3d{1.1_m, 2.2_m, 1.1_m},
+           Rotation3d{Quaternion{1.91, 0.3504, 3.3, 1.74}}};
+}  // namespace
+
+TEST(Pose3dProtoTest, Roundtrip) {
+  google::protobuf::Arena arena;
+  google::protobuf::Message* proto = ProtoType::New(&arena);
+  ProtoType::Pack(proto, kExpectedData);
+
+  Pose3d unpacked_data = ProtoType::Unpack(*proto);
+  EXPECT_EQ(kExpectedData.Translation(), unpacked_data.Translation());
+  EXPECT_EQ(kExpectedData.Rotation(), unpacked_data.Rotation());
+}