[In-App Purchases] Add environment variable for the WordPress.com API base url

[toupper]

Closes: #8016

Description

With this PR we provide the option to route the WordPress.com API requests to your wp.com sandbox e.g to use the sandbox billing mode instead of production when creation orders after an In-App purchase. That way, we avoid having to set up Charles to map the request, and ensure that all calls to create an order goes to the wp.com sandbox.
Firstly I intended to add a debug setting to switch on or off routing to the wp.com sandbox, but I ended up discarding that given the complexity of configuring all remotes to use the wp.com username. After that, I hardcoded the sandbox URL with the logged in WordPress.com username, but also rejected it because of security reasons.

Testing instructions

Prerequisites

• Have a wp.com sandbox linked to the same wp.com account you use to authenticate in the app
• Login in the terminal to that sandbox via ssh so the server can start
• Enable the wpcom-api-base-url environment variable and add your WordPress.com API base URL.

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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 pr8021-1019697 on your iPhone

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

[koke]

Nice, it works great and the implementation doesn't get any simpler 👏🏽