Add CustomerProvider abstraction

[gbirch-stripe]

Summary

Motivation

Testing

Changelog

[github-actions]

🚨 New dead code detected in this PR:

STPAPIClient+PaymentSheet.swift:16 warning: Unused function 'makeElementsSessionsParams(mode:epmConfiguration:cpmConfiguration:clientDefaultPaymentMethod:customerAccessProvider:linkDisallowFundingSourceCreation:userOverrideCountry:)'
STPAPIClient+PaymentSheet.swift:123 warning: Unused function 'retrieveElementsSession(paymentIntentClientSecret:clientDefaultPaymentMethod:configuration:)'
STPAPIClient+PaymentSheet.swift:166 warning: Unused function 'retrieveElementsSession(setupIntentClientSecret:clientDefaultPaymentMethod:configuration:)'
CustomerProvider.swift:18 warning: Assign-only property 'enabled' is assigned, but never used
CustomerProvider.swift:19 warning: Assign-only property 'initiallyChecked' is assigned, but never used
CustomerProvider.swift:24 warning: Assign-only property 'checkoutCustomerEmail' is assigned, but never used
CustomerProvider.swift:25 warning: Assign-only property 'checkoutCustomerName' is assigned, but never used
CustomerProvider.swift:26 warning: Assign-only property 'checkoutCustomerPhone' is assigned, but never used
CustomerProvider.swift:27 warning: Assign-only property 'checkoutSavedPaymentMethods' is assigned, but never used
CustomerProvider.swift:28 warning: Assign-only property 'checkoutSaveConsent' is assigned, but never used
CustomerProvider.swift:90 warning: Unused function 'make(customerAccessProvider:customerID:)'
IntentConfirmParams.swift:207 warning: Unused function 'setSetupFutureUsageIfNecessary(_:currentSetupFutureUsage:paymentMethodType:customer:)'
PaymentSheetConfiguration.swift:988 warning: Unused function 'ephemeralKeySecret(basedOn:)'

Please remove the dead code before merging.

If this is intentional, you can bypass this check by adding the label skip dead code check to this PR.

ℹ️ If this comment appears to be left in error, double check that the flagged code is actually used and/or make sure your branch is up-to-date with master.

[find-dead-code]