Create input type for NFTs

[lorisleiva]

It would be nice to have a defined type for accessing an NFT that's not just the Nft class itself.

Something like:

type NftValue = Nft | PublicKey | string;

This would allow us to have a more versatile API. For instance,

metaplex.nfts().update(nft, params);
metaplex.nfts().update(mintAddress, params);
metaplex.nfts().update("someMintAddressAsBase58", params);