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

Also add unit tests.

wpimath/src/main/native/include/frc/geometry/proto/Rotation3dProto.h

@@ -0,0 +1,18 @@
+// 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.
+
+#pragma once
+
+#include <wpi/SymbolExports.h>
+#include <wpi/protobuf/Protobuf.h>
+
+#include "frc/geometry/Rotation3d.h"
+
+template <>
+struct WPILIB_DLLEXPORT wpi::Protobuf<frc::Rotation3d> {
+  static google::protobuf::Message* New(google::protobuf::Arena* arena);
+  static frc::Rotation3d Unpack(const google::protobuf::Message& msg);
+  static void Pack(google::protobuf::Message* msg,
+                   const frc::Rotation3d& value);
+};