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

By: Yannick Lamprecht <yannicklamprecht@live.de>

paper-api/src/main/java/org/bukkit/boss/KeyedBossBar.java

@@ -0,0 +1,9 @@
+package org.bukkit.boss;
+
+import org.bukkit.Keyed;
+
+/**
+ * Represents a custom {@link BossBar} that has a
+ * {@link org.bukkit.NamespacedKey}
+ */
+public interface KeyedBossBar extends BossBar, Keyed { }