Properly re-architecture the library to handle various source abstractions

[remi-stripe]

Today, the library has multiple separate classes that "abstract" away the complexity in the API around payment sources and payout destinations.

This is partly described in details in #1066 though it was written before External Accounts existed.

This PR will attempt to rewrite or merge classes so that we end up with only three concepts:

• ExternalAccount representing the objects where funds can be paid out
• PaymentSource representing the objects acting as a Source on a Charge
• Source the existing API resource which would be a subset of PaymentSource

cc @stripe/api-libraries

• Remove the Stripe prefix from StripeBankAccount and StripeExternalAccount resources
• A Payout destination should be an ExternalAccount
• Get rid of CustomerBankAccount and move everything over to StripeBankAccount
• Add tests for ExternalAccount
• Remove Recipient-related operations on StripeCardService. Those have almost never been used in a few months and this has been deprecated for over 3 years now.
• Figure out a way to merge StripeCardCreateOptions and StripeCreditCardOptions. We-re going to punt on this for now until we can support nested options properly (sharing parameter names under a different parent parameter name)