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

Also add unit tests.

wpimath/src/main/native/include/frc/geometry/struct/Rotation3dStruct.h

@@ -0,0 +1,25 @@
+// 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/struct/Struct.h>
+
+#include "frc/geometry/Rotation3d.h"
+
+template <>
+struct WPILIB_DLLEXPORT wpi::Struct<frc::Rotation3d> {
+  static constexpr std::string_view kTypeString = "struct:Rotation3d";
+  static constexpr size_t kSize = wpi::Struct<frc::Quaternion>::kSize;
+  static constexpr std::string_view kSchema = "Quaternion q";
+
+  static frc::Rotation3d Unpack(std::span<const uint8_t, kSize> data);
+  static void Pack(std::span<uint8_t, kSize> data,
+                   const frc::Rotation3d& value);
+  static void ForEachNested(
+      std::invocable<std::string_view, std::string_view> auto fn);
+};
+
+static_assert(wpi::HasNestedStruct<frc::Rotation3d>);