Implement networking for In-app purchases

[koke]

Closes: #7835

Description

This adds a new InAppPurchasesRemote that implements the two necessary API calls for IAP:

• loadProducts to get the list of supported product identifiers from the backend
• createOrder to create a new order on the backend given an existing IAP receipt.

Testing instructions

Unit tests should pass. This code was tested within #7830 to the extent possible, since we backend pieces aren't ready yet (currently endpoints return an error).

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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 pr7875-30e2828 on your iPhone

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

[toupper]

Nice job @koke ! LGTM