fix: userCanPay without existing payment methods

[josh-burton]

Fixes #305

Describe the changes proposed

Adds a new existingPaymentMethodRequired parameter to the userCanPay method, so the developer can determine if Apple/Google pay is available without requiring cards to be added

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Additional context

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[JlUgia]

Hi @josh-burton, thank you for your contributions.
My preference would be to default to false on this new prop, in line with how both enabled providers operate today.
Also, how about we use a more straightforward prop name on the Flutter API surface? Both Apple and Google Pay have slight variations on how the API works, so I feel that something like existingPaymentMethodAvailable or similar would be more accurate, since we are looking to extract whether there's at least one payment method available for the configuration provided.

Thank you again.

[josh-burton]

Hi @josh-burton, thank you for your contributions. My preference would be to default to false on this new prop, in line with how both enabled providers operate today. Also, how about we use a more straightforward prop name on the Flutter API surface? Both Apple and Google Pay have slight variations on how the API works, so I feel that something like existingPaymentMethodAvailable or similar would be more accurate, since we are looking to extract whether there's at least one payment method available for the configuration provided.

Thank you again.

Thank you for the review. I have made those changes