Enable scoped addressing of check > data block

[radeksimko]

Context

#229 introduces the check block, which can look like this

check "health_check" {
  data "http" "terraform_io" {
    url = "https://www.terraform.io"
  }

  assert {
    condition = data.http.terraform_io.status_code == 200
    error_message = "${data.http.terraform_io.url} returned an unhealthy status code"
  }
}

That PR enables completion, hover etc. throughout the block but does not recognise the references to the scoped data block within check, i.e. data.http.terraform_io.* references above.

This was omitted from the PR as there's currently no simple way of expressing such a scoped reference through the hcl-lang schema.

Proposal

• hcl-lang: Explore ways a scoped address such as the above could be expressed in the schema
• terraform-schema: Add Address to the nested data block