Add support for Store Credits

[rainerdema]

Summary

• [x] #249

Fixes #198

This PR adds functionality to support the use of store credits when processing payments through the Stripe payment method. Specifically, the initialization of the Stripe elements and the creation of payment intents have been updated to ensure that the correct amount is charged to the user, considering any available store credits applied to the related order.

Note

• What happens if store credits fully cover the order? The Solidus Stripe Payment will be correctly invalidated, and no payment intents will be created on Stripe (same in case of using a SolidusStripe::PaymentSource)

Checklist

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• [x] I have written a thorough PR description.
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[codecov[bot]]

Codecov Report

Merging #261 (a657cad) into master (105220e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          26       26           
  Lines         484      484           
=======================================
  Hits          483      483           
  Misses          1        1           

Impacted Files	Coverage Δ
app/models/solidus_stripe/payment_intent.rb	100.00% <ø> (ø)
...lates/app/views/checkouts/payment/_stripe.html.erb	100.00% <ø> (ø)

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