feat: Make Sitemap data available via GraphQL query

[impactmass]

Resolves #4796
Impact: major
Type: feature

To be tested along with reactioncommerce/example-storefront#488

Issue

#4796
The sitemaps are generated in the core API app (ie. the reaction service), and currently available on the API domain/url. We need to make the sitemap information available through GraphQL so standalone storefronts can use it.

Solution

• Setup a GQL query for retrieving sitemap data
• Update the starterkit to use the expose data interface (PR -> feat: Setup sitemap routes to get data from GQL server example-storefront#488)

Details

• The query takes 2 params: handle and shopUrl. The handle is a string used when the sitemap was created. It can have values like sitemap.xml, sitemap-tags-1.xml.
• Sitemaps are saved with the shopId they belong to. Storefront apps like starkerkit get shopId value later on in the rendering process. To get sitemap information without depending on the rendering process, the shopUrl is passed as param.
• The shopUrl is confirmed in the GQL implementation to ensure the domain requested is for a known shop domain Shops.findOne({ domains: domain })
• The shopUrl param is also used to replace the xml placeholder: sitemap.xml.replace(/BASE_URL/g, shopUrl)

Testing

• Start reaction on this branch
• Generate Sitemaps by going to the "Shop" admin panel. Under "Options" tab, click "Refresh Sitemap"
• Start the starterkit app with this branch (feat-4796-impactmass-sitemap-endpoint).
• Visit {starterkitdomain}/sitemap.xml e.g http://localhost:4000/sitemap.xml
• You should see the sitemap xml on the page

[ @kieckhafer as the primary reviewer. @dancastellon I added you to take a pass based on the fact that you worked previously on sitemap. ]

[kieckhafer]

Works as described. IMO can be merged once @dancastellon takes a look.

[dancastellon]

Looks good!