Add const for BNPL capabilities (Affirm, Afterpay/Clearpay, Klarna)

[morksmail]

Elsewhere in our code we reference Account::CAPABILITY_CARD_PAYMENTS or Account::CAPABILITY_TRANSFERS, but there isn't one for afterpay_clearpay_payments.

[CLAassistant]

All committers have signed the CLA.

[richardm-stripe]

Thanks for submitting this. I am closing, as we removed the CAPABILITY_* constants entirely in #1341, as these constants were a bit unusual and inconsistent with the rest of the library which only provides constants for enum values.