WebAPI dynamic interface should directly accept a set of arguments

[yaakov-h]

Something like this:

var args = new Dictionary<string, object> {
	{ "key", steamApiKey },
	{ "steamid_target", steamID },
	{ "trade_offer_access_token", tradeToken ?? "" } // kept for now
};

await iEconService.GetTradeHoldDurations(args);

if (args.Length == 1 & args[0] is IDictionary....) could work. I wouldn't want any arg as a dictionary to negate the rest.

See end of #522.