Add customizable MTU support (#1068)

* Add customizable MTU support

Fixes clients being unable to connect in rare instances.

* Make config.yml nicer

bootstrap/sponge/src/main/java/org/geysermc/platform/sponge/GeyserSpongeConfiguration.java

@@ -253,6 +253,11 @@ public class GeyserSpongeConfiguration implements GeyserConfiguration {
         }
     }
 
+    @Override
+    public int getMtu() {
+        return node.getNode("mtu").getInt(1400);
+    }
+
     @Override
     public int getConfigVersion() {
         return node.getNode("config-version").getInt(0);