Get downloadable products for customer

[rogyar]

Description (*)

This PR introduces a possibility to retrieve purchased downloadable products list for a customer.

Fixed Issues (if relevant)

1. [My Account] My Downloadable Products #42: [Query] My Account > My Downloadable Products

Manual testing scenarios (*)

1. Log in as a registered customer and purchase some downloadable products (place order).
2. Generate an access token for the current user (https://devdocs.magento.com/guides/v2.0/get-started/authentication/gs-authentication-token.html#request-token)
3. Use the newly generated token in the GraphQl request header and submit the following request

{
  customerDownloadableProducts {
    items {
      date
      status
      download_url
      remaining_downloads
    }
  }
}

[ghost]

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