Burn should be an extension

[JakeHartnell]

After talking with @the-frey, I think burnable NFTs should be an extension.

Open Zepplin does this: https://docs.openzeppelin.com/contracts/4.x/api/token/erc721

Burnable is a separate extension. We could make a cw721-plus with common extensions, but I think it would be best if you could extend a cw721 with each functionality you need. For example, Open Zepplin has contract extensions like ERC721Burnable, and it's easy to add the functionality you want to an ERC721.

[JakeHartnell]

Closing for now... maybe a bit of an over optimization (burn is already supported). Feeling differently about things after #42.