Missing check for attribute assignments on config load

[malud]

Describe the bug
Since there are a lot attributes which gets evaluated at runtime, there should be checked for assignment errors on start up to prevent confusion.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run --entrypoint=/couper avenga/couper version
master 2021-03-29 acfce46

2. Provide your configuration file *.hcl. Remove sensitive data.

server "case" {
  endpoint "/**" {
    proxy {
      url = "https://httpbin.org/get/"
    }
    add_response_headers {
      x-data = "missing"
    }
  }
}

3. Provide a curl call for reproduction
   curl -v http://localhost:8080/

Expected behavior
add_response_headers is an attribute and no block. Therefore Couper should not load the configuration and points to the related misconfigured line.