[MBL-1273] Implement Create Checkout Mutation

[scottkicks]

📲 What

When the CTA on Confirm Details is pressed, call CreateCheckout

🤔 Why

The ValidateCheckout Mutation that will be called on the checkout screen needs a checkoutId returned from this mutation. We'll call it here and if successful navigate to the checkout screen, passing along the checkoutId.

🛠 How

The checkout screen isn't complete yet so there's nothing to navigate to, but we can wire this new mutation up in the meantime.

✅ Acceptance criteria

• Call mutation on CTA Tap
• If there was an error, show the error message banner
• If successful, print for now. We'll navigate later

⏰ TODO

• Navigate to Checkout screen when mbl-1210 is ready

[amy-at-kickstarter]

Presumably pledgeTotal is a double - does it need to be formatted in a specific way for the server? My concern would be that the automatic printer might output "7" or "7.0" or "7.000000001" and the server might expect a specific number of decimal places.

[amy-at-kickstarter]

Unconcerned about this one because it's an int.

[amy-at-kickstarter]

This LGTM, but only assuming that the print for pledgeTotal gives you something consistent/sane.

I'd recommend using a more specific print formatter like String.format("%.2d", pledgeTotal) or NumberFormatter so that you're guaranteed consistent results; I'd also be fine with it if there's documentation that proves this doesn't matter (either on the GraphQL side or the Swift side).

I don't know how clever the GraphQL parser is about numeric types, nor how clever Swift wants to be, I just trust neither 😉