[In-App Purchases] Route to wp.com sandbox when submitting orders

[toupper]

Closes: #8016

Description

With this PR we route the MobilePay "create orders" API request to the developer wp.com sandbox in debug mode. 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. It actually uses the user credentials to get the wp.com username, so it can compose the sandbox URL that replaces wp.com public API base URL.
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. If we need to be pointing to the wp.com sandbox to test other functionalities in the future we can extend it anyways.

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
• With a debug build, purchase an In-App purchase product and be sure that when submitting the request to our IAP backend it is pointing to your wp.com sandbox ("https://your wp.com user name.dev.dfw.wordpress.com/sandboxed-api/")

---

• 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 pr8018-efbd00b on your iPhone

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