WPCOM API: Site Settings: Additional fields for Google Analytics D8279

[allendav]

This commit syncs r165593-wpcom.
Related issue: #8181
Module change forthcoming in a separate PR: #8182

Changes proposed in this Pull Request:

• Adds a 1.4 version of the site settings endpoint that allows new fields in the google analytics setting

Testing instructions:

Fire up postman
Set your headers to
Content-Type: application/json
Authorization: X-WPCOOKIE (steal a token from a developer REST API console request)
Origin: https://developer.wordpress.com
(you'll need to use Postman Interceptor to pull that off)

First, let's double check the 1.3 endpoint for regression

Direct a v1.3 GET request at /site/{siteID}/settings ( e.g. /sites/87832313/settings )
( i.e. public-api.wordpress.com/rest/v1.3/sites/87832313/settings )
Ensure you get a wga key in the response and that it has the following sub-keys
code, anonymize_ip, ec_track_purchases, and ec_track_add_to_cart

Direct a v1.3 POST request to /site/{siteID}/settings ( e.g. /sites/87832313/settings )
Set the body to something like

{
  "wga": {
	"code": "UA-12345678-4",
	"ec_track_add_to_cart": 1
   }
}

Note: For v1 compatibility, all requests must include the code field. All other fields are optional

Now, let's test the 1.4 endpoint.

Direct a v1.4 GET request at /site/{siteID}/settings ( e.g. /sites/87832313/settings )
( i.e. public-api.wordpress.com/rest/v1.4/sites/87832313/settings )
Ensure you get a wga key in the response and that it has the following sub-keys
code, anonymize_ip, ec_track_purchases, ec_track_add_to_cart, enh_ec_tracking,
enh_ec_track_remove_from_cart, enh_ec_track_prod_impression, enh_ec_track_prod_click,
enh_ec_track_prod_detail_view, enh_ec_track_checkout_started

Direct a v1.4 POST request to /site/{siteID}/settings ( e.g. /sites/87832313/settings )
Set the body to something like

{
  "wga": {
	"code": "UA-12345678-4",
	"ec_track_add_to_cart": 1,
	"enh_ec_tracking": 1,
	"enh_ec_track_checkout_started": 1
   }
}

Note: For v1 compatibility, all requests must include the code field. All other fields are optional

Try a few combinations to ensure it works well.

cc @justinshreve