[15.x] Check if the default payment method is valid.

[srmklive]

This PR adds a method hasValidDefaultPaymentMethod which checks whether the user has a valid default payment method stored on Stripe.

Currently in cashier, we can check whether a user has a default payment method through:

• hasDefaultPaymentMethod only checks for payment type.
• defaultPaymentMethod -- only gives the default payment method details.

[github-actions]

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