Feature: Add possibility to add parameters to RPC request

[Ruhrpottpatriot]

From within UE4 I call an rpc function like this:

		auto successFunc = [](const Nakama::NRpc& rpc)
		{
			UE_LOG_ONLINE_SESSION(Log, TEXT("Session Create Success"));
		};
		Nakama::ErrorCallback errorFunc = [](const Nakama::NError& error) 
		{
			UE_LOG_ONLINE_SESSION(Error, TEXT("Session Create Error: %s"), UTF8_TO_TCHAR(error.message.c_str()));
		};

		std::string foo = R"({"owning_user_id": "Test","owning_username":"TestName","session_name": "GameSession","session_state": 0})";
		client->rpc(sessionPtr, "sessions/create", foo, successFunc, errorFunc);

In some cases there might be the need to add url parameters or headers to an RPC request. However this is currently not possible.

For example a user might have an RPC that updates data of a player-party. Showing the intent by using PUT could make api intent clearer. Another example would be a server whose response to a procedure can be a different MIME type and the client passes the type via header.