suspend fun getPaymentMethod(userId: String? = null, forceRefresh: Boolean = false): PaymentMethod?
Retrieves a user's primary payment method from a cache, database, or network.
userId - The user's UUID, required when doing a force refresh
forceRefresh - Determines if data will be refreshed with a network call.
IllegalArgumentException - Thrown when requesting a refresh without specifying a user UUID
DatabaseException - Thrown if an error occurs accessing the database
com.venuenext.vnpayment.service.UserNotFoundException - Network error thrown when user with given UUID is not found
com.venuenext.vnpayment.service.PaymentMethodException - Thrown when a network service error occurs