[In-App Purchases] Sandbox billing mode warnings

[toupper]

Closes: #8122

Description

As we had a few test IAP transactions that ended up in production because the sandbox billing mode was not enabled when testing, I wanted to propose the addition of a warning text and an alert reminder before purchasing a product in our IAP debug screen. we add a warning text instead of the IAP Debug Menu if the sandbox URL was not set in the WooCommerce Xcode schema.

Testing instructions

No WPCOM Sandbox URL in the WooCommerce schema environment variable

Open the IAP Debug screen. A warning text should appear instead of the IAP Debug menu:

WPCOM Sandbox URL in the WooCommerce schema environment variable is setup

Open the IAP Debug screen. You should see the IAP Debug Menu as usual.

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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:

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• Then installing the build number pr8123-9282886 on your iPhone

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[koke]

I don't know if I'd add an alert every single time you try to purchase. Could we look first at whether wpcom-api-base-url is set and maybe replace the whole thing with the warning if it's not? That seems less intrusive and would work as long as we don't want to use this to test in production.

[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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[toupper]

@koke Sure, please take a look now.

[koke]

Great improvement!