Add implementation of the elder Guardian API

By: md_5 <git@md-5.net>

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

@@ -1,6 +1,7 @@
 package org.bukkit.craftbukkit.entity;
 
 import net.minecraft.server.EntityGuardian;
+import net.minecraft.server.GenericAttributes;
 import org.bukkit.craftbukkit.CraftServer;
 import org.bukkit.entity.EntityType;
 import org.bukkit.entity.Guardian;
@@ -20,4 +21,30 @@ public class CraftGuardian extends CraftMonster implements Guardian {
     public EntityType getType() {
         return EntityType.GUARDIAN;
     }
+
+    @Override
+    public boolean isElder() {
+        return ((EntityGuardian)entity).cl();
+    }
+
+    @Override
+    public void setElder( boolean shouldBeElder ) {
+        EntityGuardian entityGuardian = (EntityGuardian) entity;
+
+        if (!isElder() && shouldBeElder) {
+            entityGuardian.a( true );
+        } else if (isElder() && !shouldBeElder) {
+            entityGuardian.a( false );
+
+            // Since minecraft does not reset the elder Guardian to a guardian we have to do that
+            entity.a(0.85F, 0.85F);
+            entityGuardian.aW();
+
+            // Update pathfinding (random stroll back to 80)
+            entityGuardian.bq.b(80);
+
+            // Tell minecraft that we need persistence since the guardian changed
+            entityGuardian.bW();
+        }
+    }
 }