being able to create payment intent via API

[loicginoux]

Desired Behavior

I understand that the priority is to have a payment flow working with the starter front. In order to support more cases, it would be good to also be able to create a payment intent for to a given order via API.
This would allow API clients (custom front ends, mobile apps) to use this endpoint.
suggesting a SolidusStripe::Api::IntentsController#create via the http route POST /api/orders/XXXX/stripe_intents

blocked by #154
related to #173

Current Behavior

For reference, the current payment intent creation is done in a view as said in this comment.

Additional notes

In order to support mobile apps, an ephemeral key may be created as described here

[elia]

Yes, the focus is now on the starter frontend, but by the end of this we'll have the building blocks that will make adding an api controller really straightforward.