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

By: Yannick Lamprecht <yannicklamprecht@live.de>

paper-server/nms-patches/BossBattleCustom.patch

@@ -0,0 +1,30 @@
+--- a/net/minecraft/server/BossBattleCustom.java
++++ b/net/minecraft/server/BossBattleCustom.java
+@@ -8,12 +8,27 @@
+ import java.util.UUID;
+ import java.util.function.Consumer;
+ 
++// CraftBukkit start
++import org.bukkit.boss.KeyedBossBar;
++import org.bukkit.craftbukkit.boss.CraftKeyedBossbar;
++// CraftBukkit end
++
+ public class BossBattleCustom extends BossBattleServer {
+ 
+     private final MinecraftKey h;
+     private final Set<UUID> i = Sets.newHashSet();
+     private int j;
+     private int k = 100;
++    // CraftBukkit start
++    private KeyedBossBar bossBar;
++
++    public KeyedBossBar getBukkitEntity() {
++        if (bossBar == null) {
++            bossBar = new CraftKeyedBossbar(this);
++        }
++        return bossBar;
++    }
++    // CraftBukkit end
+ 
+     public BossBattleCustom(MinecraftKey minecraftkey, IChatBaseComponent ichatbasecomponent) {
+         super(ichatbasecomponent, BossBattle.BarColor.WHITE, BossBattle.BarStyle.PROGRESS);