get_robotstxt() worker function to execute HTTP request
get_robotstxt_http_get(domain,
user_agent = utils::sessionInfo()$R.version$version.string,
ssl_verifypeer = 1)
domain from which to download robots.txt file
HTTP user-agent string to be used to retrieve robots.txt file from domain
analog to CURL option https://curl.haxx.se/libcurl/c/CURLOPT_SSL_VERIFYPEER.html -- and might help with robots.txt file retrieval in some cases