So that all child classes of BillingSession generate the same data
structure for customers that are created in Stripe, revise
`get_data_for_stripe_customer` to return a specific dataclass:
StripeCustomerData.
So that `update_or_create_stripe_customer` can work for Customer
objects with either a realm or remote_server, we create an abstract
base class, BillingSession, and implement a child class for the
current implementation of Customer objects with a realm.
Refactoring `update_or_create_stripe_customer` also moves
`create_stripe_customer` and `replace_payment_method` to the
BillingSession class.
Lauryn Menard
Aman Agrawal