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

Also add unit tests.

wpimath/src/main/native/cpp/geometry/proto/Rotation2dProto.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/Rotation2dProto.h"
+
+#include "geometry2d.pb.h"
+
+google::protobuf::Message* wpi::Protobuf<frc::Rotation2d>::New(
+    google::protobuf::Arena* arena) {
+  return google::protobuf::Arena::CreateMessage<wpi::proto::ProtobufRotation2d>(
+      arena);
+}
+
+frc::Rotation2d wpi::Protobuf<frc::Rotation2d>::Unpack(
+    const google::protobuf::Message& msg) {
+  auto m = static_cast<const wpi::proto::ProtobufRotation2d*>(&msg);
+  return frc::Rotation2d{
+      units::radian_t{m->value()},
+  };
+}
+
+void wpi::Protobuf<frc::Rotation2d>::Pack(google::protobuf::Message* msg,
+                                          const frc::Rotation2d& value) {
+  auto m = static_cast<wpi::proto::ProtobufRotation2d*>(msg);
+  m->set_value(value.Radians().value());
+}