Added a 'Custom Entity Name' feature

[AsukaRyo12]

"Hi there! I've added a 'Custom Entity Name' feature to replace the previous 'Random Number ID or Default Entity Name'.
I thought this might be useful for users who want more control over naming their entities, and I hope it adds value to the project.
If you don't mind, could you take a look at the changes? I'd love to contribute to making ZNPCsPlus even better! ❤
Thanks for your hard work on this project!"

[AsukaRyo12]

Sorry, I'm a Java beginner, I didn't check NullPointerException just now, this is my latest modified code❤

    String displayName = entity.getProperty(propertyRegistry.getByName("display_name", String.class));

    if (displayName != null) {
        EntityData<Optional<Component>> nameData = new EntityData<>(
                2,
                EntityDataTypes.OPTIONAL_ADV_COMPONENT,
                Optional.of(Component.text(displayName))
        );
        sendPacket(player, new WrapperPlayServerEntityMetadata(
                entity.getEntityId(),
                Collections.singletonList(nameData)
        ));
    }

[AsukaRyo12]

Some model rendering mods or plugins may use the entity's name to select and replace the NPC model. The previous writing method will cause the player-type NPC to always use a random number as the entity name. Therefore, in order to solve this problem, I allow NPCs to have custom names.
Now I can apply my model to the entities created by ZNpcPlus XD！