Add name and domain to account API to facilitate UX improvements

[JPry]

Changes proposed in this Pull Request:

This sets up the plugin to handle the updated API responses from the connect server. The connect server will start including name and domain information for each linked Merchant Center account.

The main changes here are to our REST API schema, and also to the Google API class. The API class renames Automattic\WooCommerce\GoogleListingsAndAds\API\Google\Proxy::get_merchant_ids() to Automattic\WooCommerce\GoogleListingsAndAds\API\Google\Proxy::get_merchant_accounts(). I've also created a new version of the get_merchant_ids() method, in case it is being used by any 3rd parties.

Changelog entry

Update – Add support for retrieving the name and domain from the Google API

[ianlin]

LGTM. Tested locally, verified the /wp-json/wc/gla/mc/accounts API returns name and domain fields in the response.

[JPry]

Just a note that these changes depend on https://github.com/Automattic/woocommerce-connect-server/pull/1844 being merged/deployed to the vip-go server.