move mint() and update_ownership() to Cw721Execute trait

[taitruong]

Saw this TODO:

cw-nfts/contracts/cw721-base/src/execute.rs

Line 80 in 177a993

// TODO pull this into some sort of trait extension??

Since it Cw721Execute covers all execute msgs, imo above methods should also be part of this trait:

cw-nfts/packages/cw721/src/traits.rs

Line 18 in 177a993

pub trait Cw721Execute<T, C>

@shanev, @larry0x, let me know whether u agree and I'll make a PR for this.