Moving OC card method logic to the new OC payment method class

[wjrosa]

Fixes STRIPE-667

Changes proposed in this Pull Request:

This PR moves all the specific Optimized Checkout logic of the card payment method to a new payment method class. The goal is to improve the code's organization and properly separate its concerns.

Testing instructions

• Checkout to this branch on your test environment (dev/moving-oc-card-method-logic-to-single-method-class)
• Connect your Stripe account
• Enable the Optimized Checkout
• Enable some payment methods
• As a shopper, add any product to your cart
• Go to the block checkout page
• Confirm the Stripe (single) payment element is displayed and you are able to complete the purchase
• Confirm the same for the classic checkout
• Confirm that you can also add payment methods

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[Mayisha]

@wjrosa I am getting some errors when testing this branch.

Settings
✅ Settings page looks good.

Shortcode checkout
✅ Single payment element is displayed on the checkout page.

🔴 Checkout fails for me with the following error-

And I see this in my logs

Block checkout
⚠️ Single payment element is displayed on the block checkout page. Probably unrelated to this PR but I don't see the test instructions on the block checkout page. But I see them in shortcode.

🔴 Checkout fails with the following error-