Store creation M2: integrate with mock IAP to test the entire flow

[jaclync]

Part of #8108

Description

Using an IAP mock

There is still an IAP backend issue (p1668419146340439-slack-C025A8VV728) that results in a Server could not read response failure (p1668592527610099/1668419146.340439-slack-C025A8VV728) when I tested purchasing the eCommerce plan. In order to unblock us from testing and developing the store creation flow, I decided to create a mock implementation of InAppPurchasesForWPComPlansProtocol in debug builds using the DEBUG macro and a feature flag storeCreationM2WithInAppPurchasesEnabled to easily turn it on and off. The DEBUG macro is used so that the mock struct isn't included in non-debug builds, since it's generally a good practice not to include mock/debug classes in production builds. If you feel like the use of the macro makes it harder to read, I can also remove the macro condition since it's only temporary.

Changes to navigation bar title & back button styles

Because the store name form (StoreNameForm) and plan purchase view (StoreCreationPlanView) screens do not use any navigation bar title, before this PR a gap is shown below the navigation bar for these screens. I struggled with various navigation bar title styles in all different screens in the flow, sometimes disabling the large title style in one screen results in the next screen not being able to show a large title. I also got stuck on hiding the back button in the navigation bar for StoreCreationPlanView. After debugging for a while, I realized that the issues were from having SwiftUI view in a UIHostingController and setting navigation bar title and back button in the hosting controller 🤦🏻‍♀️ As a result, I changed the navigation bar title and back button configurations from the hosting controller to the SwiftUI view.

Changes to StoreCreationCoordinator

Using the IAP interface InAppPurchasesForWPComPlansProtocol, we can now integrate the whole flow with each step in place. To wait for the newly created site to become a Jetpack site, I copied most of the logic from M1 implementation but the site ID subscription feels a bit odd to me since it never changes. Please feel free to suggest a better solution not using Combine.

Testing instructions

• Log in if needed
• Go to the Menu tab, and tap Switch store
• On the store picker, tap + Add a store
• Tap Create a new store --> an in-progress modal should be shown briefly, then dismissed. the store name form is then presented
• Enter some store name and tap Continue when ready --> the store name should be used for the initial domain query in the next screen
• Search for another domain if you like, and select one domain to Continue --> a free Simple site should be created, and a store summary screen is shown with the store name and site slug
• Tap Continue to payment --> the eCommerce plan should be shown
• Tap Create Store for $69.99/month --> an in-progress screen should be shown about creating the store. after the site becomes available in the /me/sites response, the success screen should be shown
• Tap Manage My Store to continue to the new site in the app --> a WC version alert is probably shown because it's a Simple site without WC

Screenshots

simulator.mov

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8154-9fdf543 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[selanthiraiyan]

LGTM 🚢 🎉

[selanthiraiyan]

Thanks for explaining in the comments.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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