Allow hidden blocks to be revealed by plugins (#4017)

* Add revealedBlocks rule

(1/2)

* Use revealedBlocks rule

(2/2)

core/src/mindustry/game/Rules.java

@@ -80,6 +80,8 @@ public class Rules{
     public Seq<WeatherEntry> weather = new Seq<>(1);
     /** Blocks that cannot be placed. */
     public ObjectSet<Block> bannedBlocks = new ObjectSet<>();
+    /** Reveals blocks normally hidden by build visibility */
+    public ObjectSet<Block> revealedBlocks = new ObjectSet<>();
     /** Unlocked content names. Only used in multiplayer when the campaign is enabled. */
     public ObjectSet<String> researched = new ObjectSet<>();
     /** Whether ambient lighting is enabled. */