Allow user to upload a custom image and metadata for an NFT that they mint

[TheRook]

Allow the user to buy an NFT at the current ask price. This will be added to their flake.art wallet, and they can choose to upload an image and set the name and description - or take one of our images to finish minting the NFT.

1. The backend will store the uploaded image to https://flake.art/i/{index}, using the index of the NFT with no file extension.
2. https://flake.art/n/{index} - show the NFT metadata (HIP-10: NFT Metadata JSON Schema hiero-ledger/hiero-improvement-proposals#50)