In-app purchases debug screen

[koke]

Closes: #7833
To be merged after #7851

Description

In #7832, we added a feature flag to enable IAP in development builds. However, we still don't have the right place to hook this up in site creation, so this adds a new entry in the Hub menu where we can add a UI to test the IAP features.

Because this is more of a developer tool than an in-progress final feature, I'm hiding this behind a beta feature so it's disabled by default.

Testing instructions

1. Go to the hub menu, no [Debug] IAP item should appear
2. Go to Settings > Experimental features
3. Enable In-app purchases
4. Go to the hub menu, a [Debug] IAP item should appear
5. Tapping on [Debug] IAP should show a No products available message (for now).

Screenshots

Hub menu	IAP Debug	Experimental Features

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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 pr7870-794b199 on your iPhone

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

[toupper]

It tests well and the code looks good! 🎉