Add metadata to KtdGridLayoutItem

[danilobassi8]

Hi! First of all, congratulations on this great library.

I was building a dynamic dashboard where users can add components based on the widgets they choose to display. During the implementation, I noticed there wasn’t a property available to define which component should be rendered (I’m persisting the grid state in localStorage).

I saw that there is already an open issue for this (#123), so I decided to add metadata support to grid items to address this limitation.

I also updated the playground example to verify that the metadata works correctly.
With this change, users can now attach and manage their own custom data within grid items.

Please let me know if you’d prefer me to revert the playground changes. I included them only to ensure everything was working as expected.