Title: Router check tool could check for absence or presence of a header, regardless of header value.

Description:
router_check_tool can verify HTTP header values, but not explicitly test for http header absence. There's a workaround to test for empty value string, which is nearly but not quite the same thing.
Adding present: true/false to the request_header_fields and response_header_fields would allow for testing for value-agnostic presence of a header, and for explicit absence of a header.

validate:
  response_header_fields:
    - key: content-length
      present: true

validate:
  response_header_fields:
    - key: x-ride-corp-internal
      present: false