Added AccountGroupUpdate hook, when trying to change a user group.

[AgaSpace]

Added a hook to change the user group. The hook is in AccountHooks and is named AccountGroupUpdate.
The event takes AccountGroupUpdateEventArgs as an argument, but it can be represented as AccountGroupUpdateByPlayerEventArgs in case a user group is updated by some player.

Examples of use:

public void OnAccountGroupUpdate(AccountGroupUpdateEventArgs args)
{
	if (args.AccountName == "Zoom L1" && args is AccountGroupUpdateByPlayerEventArgs)
	{
		TSPlayer author = ((AccountGroupUpdateByPlayerEventArgs)args).Player;
		if (author.Account.Name != "Zoom L1")
			args.Handled = true;
	}
	else
	{
		TShock.Log.ConsoleInfo($"The user group {args.AccountName} has been updated.");
	}
}

public void OnAccountGroupUpdate(AccountGroupUpdateEventArgs args)
{
	if (args is AccountGroupUpdateByPlayerEventArgs)
	{
		TSPlayer author = ((AccountGroupUpdateByPlayerEventArgs)args).Player;
		UserAccount account = TShock.UserAccounts.GetUserAccountByName(args.AccountName);
		if (account != null) // Must be used, because the user can be null.
		{
			if (account.Group.ToLower() == "superadmin" && author.Group is not SuperAdminGroup)
				args.Handled = true;
		}
	}
}

[Arthri]

The exception message is confusing, The user John group could not be updated