SFMLHelper: Add stream insertion overload helpers for enum types

Will be used in future changes to eliminate the need to cast when
inserting various message IDs

Source/Core/Common/SFMLHelper.h

@@ -3,18 +3,37 @@
 
 #pragma once
 
+#include <type_traits>
+
+#include <SFML/Network/Packet.hpp>
+
 #include "Common/CommonTypes.h"
 #include "Common/Swap.h"
 
-namespace sf
-{
-class Packet;
-}
-
 sf::Packet& operator>>(sf::Packet& packet, Common::BigEndianValue<u16>& data);
 sf::Packet& operator>>(sf::Packet& packet, Common::BigEndianValue<u32>& data);
 sf::Packet& operator>>(sf::Packet& packet, Common::BigEndianValue<u64>& data);
 
+template <typename Enum, std::enable_if_t<std::is_enum_v<Enum>>* = nullptr>
+sf::Packet& operator<<(sf::Packet& packet, Enum e)
+{
+  using Underlying = std::underlying_type_t<Enum>;
+  packet << static_cast<Underlying>(e);
+  return packet;
+}
+
+template <typename Enum, std::enable_if_t<std::is_enum_v<Enum>>* = nullptr>
+sf::Packet& operator>>(sf::Packet& packet, Enum& e)
+{
+  using Underlying = std::underlying_type_t<Enum>;
+
+  Underlying value{};
+  packet >> value;
+
+  e = static_cast<Enum>(value);
+  return packet;
+}
+
 namespace Common
 {
 u64 PacketReadU64(sf::Packet& packet);

Source/Core/Core/NetPlayProto.h

@@ -121,7 +121,7 @@ struct NetTraversalConfig
 };
 
 // messages
-enum
+enum MessageID : u8
 {
   NP_MSG_PLAYER_JOIN = 0x10,
   NP_MSG_PLAYER_LEAVE = 0x11,