Pass Leaderboard Id to score JSON objects

[SkaterDad]

Hello!

I came across a use case for getting leaderboard scores, and noticed the Leaderboard ID was not part of the callback data.

This PR proposes to pass along the String leaderboardId with the LeaderboardScore, and adding a leaderboard_id key to the JSON object. This allows us to know which leaderboard we're dealing with in Lua.

I've tested this on a published game with my phone. The changes are simple, so I expect it will not have issues.

Example:

-- somewhere, querying 2 leaderboards

	gpgs.leaderboard_get_player_score(LEADERBOARD_ID, gpgs.TIME_SPAN_ALL_TIME, gpgs.COLLECTION_PUBLIC)
	gpgs.leaderboard_get_player_score(HIGHLEVEL_ID, gpgs.TIME_SPAN_ALL_TIME, gpgs.COLLECTION_PUBLIC)

-- in the callback

	elseif message_id == gpgs.MSG_GET_PLAYER_SCORE then
		if not message.error and message.score then
			local gpgs_score = tonumber(message.score)

			-- If GPGS high score > current game score, apply it!
			if message.leaderboard_id == LEADERBOARD_ID then
				if gpgs_score > savedata.stats.high_score then
					savedata.update_score(gpgs_score)
				end
			-- If GPGS high level > current game level, apply it
			elseif message.leaderboard_id == HIGHLEVEL_ID then
				if gpgs_score > savedata.stats.high_level then
					savedata.update_level(gpgs_score)
				end
			end
		end

[britzl]

Good improvement! Thanks.