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/entity/Boss.java

@@ -0,0 +1,16 @@
+package org.bukkit.entity;
+
+import org.bukkit.boss.BossBar;
+
+/**
+ * Represents the Boss Entity.
+ */
+public interface Boss extends Entity {
+
+    /**
+     * Returns the {@link BossBar} of the {@link Boss}
+     *
+     * @return the {@link BossBar} of the entity
+     */
+    BossBar getBossBar();
+}