Fetch the store's product settings

[bummytime]

As part of the product detail screen (ref: #764 for WCiOS), @nbradbury astutely pointed out that the dimensions we are receiving from the products API doesn't have units attached to measurements. On the web, the WC settings UI contains these values:

So much like we did for general site settings in #369, #377, and #410 we should grab the product settings via

GET https://public-api.wordpress.com/rest/v1.1/jetpack-blogs/$siteID/rest-api/?path=%2Fwc%2Fv3%2Fsettings%2Fproducts

and stash these values into Storage for use elsewhere in the app. The good news is that we all we need to do is:

• Add a new func to SiteSettingsRemote to fetch the product settings (from that API call above) and return [SiteSetting]
• Update SettingStore.retrieveSiteSettings() to also include the new func call in SiteSettingsRemote ☝️
• Use the newly-captured product settings on the product detail screen

SiteSettingsMapper can be reused in the new SiteSettingsRemote func and the settings' models + Storage logic should not have to be updated since they are designed to handle generic setting objects.