fix(fastly_vcl_service): ensure backend names are unique

[Integralist]

This PR ensures the following config (which incorrectly sets the same value for the name attribute) will report an error...

  backend {
    address = "httpbin.org"
    name    = "httpbin"
  }

  backend {
    address = "example.com"
    name    = "httpbin"
  }