Add API to manipulate boss bar of entities and those created by commands

By: Yannick Lamprecht <yannicklamprecht@live.de>

paper-server/nms-patches/EnderDragonBattle.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/EnderDragonBattle.java
++++ b/net/minecraft/server/EnderDragonBattle.java
+@@ -21,7 +21,7 @@
+ 
+     private static final Logger a = LogManager.getLogger();
+     private static final Predicate<Entity> b = IEntitySelector.a.and(IEntitySelector.a(0.0D, 128.0D, 0.0D, 192.0D));
+-    private final BossBattleServer c;
++    public final BossBattleServer c; // PAIL private -> public, rename bossBattleServer
+     private final WorldServer d;
+     private final List<Integer> e;
+     private final ShapeDetector f;