Add custom save file support for serializable current run data.

[lamali292]

This PR introduces support for per-mod saves of current runs.

Each mod now can register its own isolated save file stored alongside the base run save.

current_run.save
mods/<mod_id>/current_run.save

To use the new system, tag a static field in your mod with the [ModSave] attribute. The system will automatically handle the loading and saving of this field during the standard run persistence cycle.

public static class MySaveManager
{
    [ModSave] public static MyRunData RunData = new();
}

Your save class should implement ISaveSchema and IPacketSerializable to maintain compatibility with the existing serialization engine. This structure mirrors the internal SerializableRun used by the base game.

public class MyRunData : ISaveSchema, IPacketSerializable
{
    [JsonPropertyName("schema_version")]
    public int SchemaVersion { get; set; } = 1;
    
    [JsonPropertyName("player_data")]
    public List<MyPlayerData> PlayerData { get; set; } = [];
    
    public void Serialize(PacketWriter writer)
    {
        writer.WriteInt(SchemaVersion);
        writer.WriteInt(PlayerData.Count);
        foreach (var pData in PlayerData)
        {
            pData.Serialize(writer);
        }
    }

    public void Deserialize(PacketReader reader)
    {
        SchemaVersion = reader.ReadInt();
        var count = reader.ReadInt();
        PlayerData = [];
        for (var i = 0; i < count; i++)
        {
            var pData = new MyPlayerData();
            pData.Deserialize(reader);
            PlayerData.Add(pData);
        }
    }
}

You can nest any custom data types within your schema, provided they implement IPacketSerializable.

public class MyPlayerData: IPacketSerializable
{
    [JsonPropertyName("net_id")]
    public ulong NetId { get; set; }

    [JsonPropertyName("saved_cards")]
    public List<SerializableCard> SavedCards { get; set; } = [];

    [JsonPropertyName("my_saved_value")]
    public int SavedValue{ get; set; }
    
    public void Serialize(PacketWriter writer)
    {
        writer.WriteULong(NetId);
        writer.WriteInt(SavedValue);
        writer.WriteList(SavedCards );
    }

    public void Deserialize(PacketReader reader)
    {
        NetId = reader.ReadULong();
        SavedValue= reader.ReadInt();
        SavedCards = reader.ReadList<SerializableCard>();
    }
}

(My downfall save data is no longer in Relics, yay! Maybe this can be reused here. Sorry for the many pull requests, but these are features that might be beneficial.)

• Make it completly multiplayer safe

[lamali292]

and obviously access it like this

public static MyPlayerData FromPlayer(Player player)
{
    var netId = player.NetId;
    var data = MyRunData.PlayerData.Find(p => p.NetId == netId);
    if (data != null) return data;
     data = new MyPlayerData { NetId = netId };
     MyRunData.PlayerData.Add(data);
     return data;
}

public static List<CardModel> GetSavedCards(Player player) =>
       MyPlayerData.FromPlayer(player).SavedCards.Select(CardModel.FromSerializable).ToList();

public static void AddSavedCard(Player player, CardModel card) =>
       MyPlayerData.FromPlayer(player).SavedCards.Add(card.ToSerializable());