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

Also add unit tests.

wpimath/src/main/native/cpp/geometry/proto/Rotation3dProto.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 "frc/geometry/proto/Rotation3dProto.h"
+
+#include "geometry3d.pb.h"
+
+google::protobuf::Message* wpi::Protobuf<frc::Rotation3d>::New(
+    google::protobuf::Arena* arena) {
+  return google::protobuf::Arena::CreateMessage<wpi::proto::ProtobufRotation3d>(
+      arena);
+}
+
+frc::Rotation3d wpi::Protobuf<frc::Rotation3d>::Unpack(
+    const google::protobuf::Message& msg) {
+  auto m = static_cast<const wpi::proto::ProtobufRotation3d*>(&msg);
+  return frc::Rotation3d{
+      wpi::UnpackProtobuf<frc::Quaternion>(m->q()),
+  };
+}
+
+void wpi::Protobuf<frc::Rotation3d>::Pack(google::protobuf::Message* msg,
+                                          const frc::Rotation3d& value) {
+  auto m = static_cast<wpi::proto::ProtobufRotation3d*>(msg);
+  wpi::PackProtobuf(m->mutable_q(), value.GetQuaternion());
+}