Preloading RSC from external URLs (#10095)

Ports the external URL preloading from tg-station. This allows us to point the config to an external URL, from which the .rsc file will be downloaded. This will be way faster than using the BYOND system, as we don't have to deal with BYOND's netcode.

code/controllers/configuration.dm

@@ -316,6 +316,8 @@ var/list/gamemode_cache = list()
 	var/profiler_restart_period = 120 SECONDS
 	var/profiler_timeout_threshold = 5 SECONDS
 
+	var/list/external_rsc_urls = list()
+
 /datum/configuration/New()
 	var/list/L = typesof(/datum/game_mode) - /datum/game_mode
 	for (var/T in L)
@@ -957,6 +959,9 @@ var/list/gamemode_cache = list()
 				if ("forumuser_api_key")
 					global.forumuser_api_key = value
 
+				if ("external_rsc_urls")
+					external_rsc_urls = splittext(value, ",")
+
 				else
 					log_misc("Unknown setting in configuration: '[name]'")