API for an entity's scoreboard name

Was obtainable through different methods, but you had to use different
methods depending on the implementation of Entity you were working with.

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java

@@ -1259,4 +1259,11 @@ public abstract class CraftEntity implements org.bukkit.entity.Entity {
         return !this.getHandle().level().noCollision(this.getHandle(), aabb);
     }
     // Paper end - Collision API
+
+    // Paper start - entity scoreboard name
+    @Override
+    public String getScoreboardEntryName() {
+        return this.getHandle().getScoreboardName();
+    }
+    // Paper end - entity scoreboard name
 }