Domain settings: networking layer changes for 2 endpoints

[jaclync]

Part of #8558

Description

Why

After the user creates a store, they might want to choose a proper domain for their new store. The domain settings screen shows the domains of a WPCOM site, and allows the merchant to add a domain either using their domain credit or the checkout flow similar to the Jetpack app.

Networking layer changes

More technical details at the spike: pe5sF9-Ug-p2

This PR just contains the networking layer changes, since the JSON response files were pretty large. There are two endpoints required to show the domain settings UI:

• DomainRemoteProtocol.loadDomains: this loads the site's domains, including the free staging domain
• PaymentRemoteProtocol.loadSiteCurrentPlan: this loads the site's WPCOM plans, and returns the current plan to determine if the site has domain credit so that the user can pick a domain for free (available from certain WPCOM plans)
  • The WPCOMPlan field of WPComSitePlan was removed since it requires extra manual parsing due to the API response, and we're not using this field for the foreseeable future

Some manual decoding was required for both endpoints since the API response isn't straightforward, I added comments in the code.

Testing instructions

The changes aren't used in the Yosemite layer yet, they will be in the next PR. Just CI is sufficient.

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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 pr8600-d4330c3 on your iPhone

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

[selanthiraiyan]

LGTM 🚢