Add the Account field to BankAccount

[pauldemarco]

This change adds the already present (network-wise) Account field to the BankAccount struct, and allows the BankAccount to become self contained --no longer needing to tuple an Event struct alongside the BankAccount when handling webhooks.

Tested working for sc.BankAccounts.New and when responding to a account.external_account.updated event in a webhook.

[remi-stripe]

Thanks a lot for starting the work here! I left a comment as something needs to be changed before we can merge!

[remi-stripe]

Thanks for the quick change, and for doing all the work!