Implement access credentials request for URL XRefs

[liutec]

If content-type is application/json and HTTP Code 401 received when fetching the content of an URL XRef, assume that the content is JSON and has the following structure:

{
  "cli": {
    "message": "Enter your credentials", (optional, default "Authentication required")
    "request": {
      "<variable_name>": {
        "prompt": "Enter <variable_name>", (optional, default "Enter variable_name")
        "type": "string/password", (optional, default string)
        "default": "value" (optional, default empty)
      }
    },
    "order": [
      "<variable_name>"
    ]
  }
}

Ask the user to enter each requested variable in the specified order.
After the user has entered all requested variables, post them back as JSON encoded to the XRef's URL. (eg. {"<variable_name>": "value"}
If the "request" key is empty, assume fetching has failed.