Add DSM checkout

[walmat]

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name: Add DSM Checkout Process
about: Integrates DSM UK / NY / SG / JP into task runner checkout process

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Changes

• Added frontend functions to checkout.js
  • AddToCart: handles frontend add to cart
  • ProceedToCheckout: handles a call to create the checkout session
  • ShippingRates: handled a if/else case to do frontend mode if necessary
  • SubmitCustomerInformation: handles the post to submit customer information
• Small bug fixes in other places

Checks

• CI passes
• Coverage (<2%∆)
• Manual Checks (when each site is live)
  • Run a task in variant / keyword / link mode
    • Right now, the task should never resolve due to Captcha Harvester "ERROR for site owner: Invalid domain for site key" #204
    • Verify the task gets stuck in the captcha loop

fixes #238

[pr1sm]

After viewing these changes, I made a clarifying comment on the #238 thread.

I'm a little wary accepting the proposed changes, because the level of complexity of the checkout class increases quite a bit, while the readability drops. Combining both checkout methods together into one file/class is very confusing and I'm not sure it's the right solution to the issue.

I'm not sure when the next drop is coming, but if we have time, I would prefer we keep the two checkout methods split into separate files to reduce confusion.

[walmat]

closing...