NetPlay: Refactor some functions into a common header

Source/Core/Core/NetPlayCommon.h

@@ -0,0 +1,21 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <SFML/Network/Packet.hpp>
+
+#include <array>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "Common/CommonTypes.h"
+
+namespace NetPlay
+{
+bool CompressFileIntoPacket(const std::string& file_path, sf::Packet& packet);
+bool CompressBufferIntoPacket(const std::vector<u8>& in_buffer, sf::Packet& packet);
+bool DecompressPacketIntoFile(sf::Packet& packet, const std::string& file_path);
+std::optional<std::vector<u8>> DecompressPacketIntoBuffer(sf::Packet& packet);
+}  // namespace NetPlay