feat: EOA support for the Checkout component

[azf20]

What changed? Why?

This introduces support for sequential approve / call actions as a fallback to the experimental wagmi writeContracts, to enable EOAs to use the component

Currently the component does not work for EOAs, as it relies on experimental wagmi multicall functionality:

• If an EOA is connected via Coinbase wallet, it will fail trying to checkCapabilities
• If an EOA is connected in another way (e.g. Metamask), the user will be prompted to connect a Coinbase wallet, rather than leveraging the existing connection

This is confusing and limits the usefulness of the component. By adding support for sequential transactions, this can be used for all account types.

There is a small optimisation to check the allowance before making the "approve" transaction, in case that can be skipped (e.g. if the approve was made already and then the purchase wasn't made for some reason, or if there is an infinite / higher approval).

Notes to reviewers

Currently this only adds the feature (to unblock EthGlobal hackathon progress), but can add tests / docs if there is appetite to merge this upstream.

How has it been tested?

• Checked / linted / built
• Tested with Metamask, Coinbase wallet (passkey) and Coinbase wallet (app, EOA)

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