Pay - unified interface

[BellringerQuinn]

Now that we have #288, #289, #290, and #291, a user can purchase an ERC1155 using their credit card, either as a Primary Sale (via Primary Sales contract) or as a secondary sale (via marketplace v2 API); and they can do so using either Sardine or Transak as vendors. Let's combine all of these under one unified interface.

Please keep in mind, that we will likely be adding additional checkout providers like Sardine and Transak in the future. It should be easy to add or remove a vendor. It is recommended to checkout the IFiatPay interface, SequencePay implementation, and IFiatPayFactory interface for some ideas on how to do this.

There is also an ERG for the marketplace + Pay system in the Unity SDK that gives an overview of the Pay system that may be helpful for this
https://github.com/0xsequence/sequence-unity/blob/Feature/marketplaceCheckout/Packages/Sequence-Unity/Sequence/SequenceSDK/Marketplace/ERG_CheckoutModal.mmd