CMS block coverage

[paliarush]

As a Magento Developer, I want to query for CMS block content using the GraphQL API so that I can build basic ecommerce storefronts using only GraphQL for efficiency.

Use cases:

• Developer can query GraphQL for CMS block content and get back all necessary data to render the page on storefront or in 3rd party applications.

Acceptance Criteria

• Only enabled blocks will be returned by the API
• Store view context is passed in the HTTP header and only blocks relevant to given context will be returned
• Staging versions of the page cannot be queried - only the currently active blocks are returned
• [DRAFT] API returns the following info
  • title
  • identifier
  • rendered (with no Magento-specific directives) or un-rendered block content, depending on configuration

Changes should be covered with web API functional tests. See \Magento\GraphQl\Catalog\ProductViewTest::testQueryAllFieldsSimpleProduct as an example.