Implement support for deferred PaymenIntent confirmation

[elia]

Summary

Fixes #209, Fixes #185, Fixes #190

This PR improves the payment process by replacing the double flow of Payment Intent/Setup Intent with a Payment Intent flow in a specific scenario described in the Stripe documentation: Build a two-step confirmation experience

With the new approach, the two-step subdivision achieves the following issues:

• The creation of the Stripe payment method remains in the payment step (as before)
• The Payment intent creation will be invoked during the confirm step rendering.
• The payment authorization occurs when the order is submitted on the confirm page.

The benefits of this update include:

• Removing compromises in the payment intent flow that previously required "skipping" the confirm step due to the premature authorization executed in the payment step.
• Aligning the authorization process with the Solidus checkout flow.
• Removing the authorize and purchase methods from the gateway, as they are no longer needed.

Checklist

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[codecov[bot]]

Codecov Report

Merging #249 (1cab1dc) into master (7396336) will decrease coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
- Coverage   99.79%   99.77%   -0.03%     
==========================================
  Files          25       24       -1     
  Lines         487      442      -45     
==========================================
- Hits          486      441      -45     
  Misses          1        1              

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	100.00% <ø> (ø)
app/models/solidus_stripe/payment_method.rb	100.00% <ø> (ø)
lib/solidus_stripe/testing_support/factories.rb	100.00% <ø> (ø)
...p/controllers/solidus_stripe/intents_controller.rb	100.00% <100.00%> (ø)
app/models/solidus_stripe/payment_intent.rb	100.00% <100.00%> (ø)
...lates/app/views/checkouts/payment/_stripe.html.erb	100.00% <100.00%> (ø)
...tes/app/views/orders/payment_info/_stripe.html.erb	100.00% <100.00%> (ø)
lib/solidus_stripe/engine.rb	100.00% <100.00%> (ø)

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[elia]

I'm also wondering if we need the SolidusStripe::PaymentIntent model at all. It's not needed from the webhook handlers, as the response_code is filled on payment intent creation. Can you help me understand? Is it only needed in the case a user refreshes the confirm page?

@waiting-for-dev sorry, I merged before replying to this question. And yes, it's there to ensure we have a single payment intent for each order. As you pointed out there might be indirect ways to do the same through Payment#response_code but as we discussed when we introduced the class this way it should be more reliable.