Link SolidusStripe::Customer to reusable payment sources

[elia]

What

Associate it to the stripe customer id that was created for the order's user whenever a payment source is created for a reusable profile.

See also https://github.com/solidusio/solidus_braintree/blob/c2c6452bb54244afce6af4f27d81b081da10bff6/app/models/solidus_braintree/gateway.rb#L227-L249.

Dependencies

• Blocked by Add support for reusable payment sources #153
• Blocked by Track Stripe Customers with a SolidusStripe::Customer model #163

Why

Payments should be tied to a single customer on stripe as they're coming from a single user on solidus, this will make handling issues and investigations much easier (e.g. from customer support team members).

[loicginoux]

This might be for another ticket, let me know but when having a customer created and linked to a payment intent, from my understanding of Stripe, iOS and Android clients will also need an ephemeral key as described in the Stripe documentation
Something like:

ephemeral_key = Stripe::EphemeralKey.create({
  customer: customer['id'],
}, { stripe_version: Stripe.api_version})

and then the PaymentIntentController#create will respond with all the info need o that any kind of api client can handle it

render json: { 
  client_secret: intent.client_secret,
  ephemeral_key: ephemeral_key['secret'],
  customer: customer['id'],
}

[elia]

For serving iOS clients it's probably best to add a dedicated controller under Spree::Api and return what's needed there.

In the latest branch (which we'll hopefully merge soon) the secret is not fetched via Ajax anymore and it's instead being provided by to the payment view directly.

[elia]

@loicginoux I extracted your comment about iOS/android to #207, so we don't lose track

[elia]

Fixed in #172