Replace the cw721::CustomMsg trait with cosmwasm_std::CustomMsg

[larry0x]

In packages/cw721/src/traits.rs is this TODO item:

// TODO: move this somewhere else... ideally cosmwasm-std
pub trait CustomMsg: Clone + std::fmt::Debug + PartialEq + JsonSchema {}

In fact, cosmwasm-std already has the CustomMsg trait. This PR replaces cw721::CustomMsg trait with cosmwasm_std::CustomMsg.

[0xekez]

@larry0x looks like this needs a rebase before it can be merged