Update ERC-7683: 'open' and 'fill' methods should be payable

[Zodomo]

Making the open methods payable allows for natives to trivially be used in user requests. Native payment can easily be accounted for alongside ERC-20 tokens by using address(0). Adding the payable modifier will make it much easier for integrators to account for native payments. This logic also follows for fill, as fillers might need to provide native payment to the settlement contract to fulfill the user's order. It seems like an anti-pattern to require handling native payment in functions outside of the spec.

[eip-review-bot]

File ERCS/erc-7683.md

Requires 1 more reviewers from @marktoda, @mrice32, @nicholaspai

[github-actions[bot]]

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