English-auction: Refactor to use dynamic storage types in structs

There are several `TODOs` outlining areas where refactoring of the existing code can now be performed, as of the implementation of https://github.com/FuelLabs/sway-rfcs/blob/05b7594b0b64ad91baaedb732994fc3f251b0042/rfcs/0008-storage-handler.md

These `TODOs` are in:
- `project/contracts/auction-contract/src/main.sw`
- `project/contracts/auction-contract/src/data_structures/nft_asset.sw`