Currently we have our own nft_metadata storage which we use for assigning metadata to tokens. PSP34Metadata provides it's own solution for storing metadata (attributes), which we are using for collections but not tokens.

This PR removes our custom nft_metadata storage, and instead uses the PSP34Metedata attribute implementation. The benefit here is we can access token metadata via the public ink PSPS34Metadata_get_attribute function, as well as the provided token_uri() function.

I have also fixed a discrepancy where token_uri was accepting a u64 instead of an Id type.