Return full canonical product URL within product object

[paliarush]

As a Magento developer, I want GraphQL product data to include the canonical product URL so that it's easy for me to render links to any product.

Canonical URL resolution is already implemented in \Magento\UrlRewriteGraphQl\Model\Resolver\UrlRewrite::resolve, which can be used a reference.

See how to expose a new query via GraphQL in Magento

Acceptance criteria:

1. canonical_url field is added to ProductInterface GraphQL interface. See Magento/CatalogGraphQl/etc/schema.graphqls
2. Changes are covered with web API functional tests. See \Magento\GraphQl\Catalog\ProductViewTest::testQueryAllFieldsSimpleProduct as an example.

[paliarush]

Merged in PR #7