Add "may transfer" post-condition for NFTs

[brice-stacks]

Similar to how a fungible token post-condition can specify that less than or equal to some amount may be transferred, NFTs should have the same type of optional allowance. Currently, you can only specify that an NFT must be sent or must not be sent and you cannot specify that it may be sent. There are situations in DeFi applications where this behavior is needed to allow users to use deny mode and properly protect their transactions.

[brice-stacks]

I believe this can be done by adding a MaybeSent to the condition code for NFTs, something like:

pub enum NonfungibleConditionCode {
    Sent = 0x10,
    NotSent = 0x11,
    MaybeSent = 0x12,
}