function isExtRef(obj, key) {
  return key === '$ref' &&
    typeof obj[key] === 'string' &&
    obj[key] !== undefined &&
    !obj[key].startsWith(localPointer) &&
    !stringIsAValidUrl(obj[key]);
}
Determines if a value of a given key property of an object
is an external reference with key $ref and value that does not start with #
@param {object} obj - parent object of the $ref property
@param {string} key - property key to check
@returns {boolean} - true if the property key is $ref and the value does not start with #
otherwise false