Add Server.MapPool setting for dedicated servers.

This takes a list of map UIDs which may be locally installed or hosted
on the resource center. If any maps aren't found, startup will be
delayed by up to 10 seconds while it attempts to query the resource
center.

OpenRA.Game/Settings.cs

@@ -102,6 +102,9 @@ namespace OpenRA
 		[Desc("For dedicated servers only, treat maps that fail the lint checks as invalid.")]
 		public bool EnableLintChecks = true;
 
+		[Desc("For dedicated servers only, a comma separated list of map uids that are allowed to be used.")]
+		public string[] MapPool = Array.Empty<string>();
+
 		[Desc("Delay in milliseconds before newly joined players can send chat messages.")]
 		public int FloodLimitJoinCooldown = 5000;