Add offchain bid auction contract variant

[cpubot]

Develop a variant of the auction contract which allows bidders to bid with both XTZ and credit card. This contract should expose an additional entrypoint which allows the admin to override the current (high) bid.

On the front-end, upon placing a credit card bid, the dApp should auth the user's credit card, ensuring that the provided card is billable. Because bids must denominated in the XTZ currency, the dApp should should send along the USD valued bid denominated in XTZ at the time of calling the offchain bid entrypoint so that the contract does not need to keep track of different bid currencies. However, the contract likely should keep track of whether or not the high bid is a credit card bid so that it can adjust its logic elsewhere appropriately.

Upon auction end, if the high bidder is a credit card bidder, the dApp should bill the provided card. Assuming the billing is successful, the auction can be resolved. If the billing is not successful, the auction should be cancelled (and NFT distributed back to the admin).