Hidden field added to metadata response

We added a new field, `hidden`, to our [NFT metadata](https://docs.center.dev/reference/shownftmetadatabatch-v2) response.

We added a new field, hidden, to our NFT metadata response.

This field indicates if the collection is hidden on OpenSea - which is often a good indication of spam. We will be rolling out additional spam detection features in the coming weeks.