curl: add --rate to set max request rate per time unit

--rate "12/m" - for 12 per minute or
--rate "5/h" - for 5 per hour

Removed from TODO

Closes #8671

src/tool_listhelp.c

@@ -559,6 +559,9 @@ const struct helptxt helptext[] = {
   {"-r, --range <range>",
    "Retrieve only the bytes within RANGE",
    CURLHELP_HTTP | CURLHELP_FTP | CURLHELP_SFTP | CURLHELP_FILE},
+  {"    --rate <max request rate>",
+   "Request rate for serial transfers",
+   CURLHELP_CONNECTION},
   {"    --raw",
    "Do HTTP \"raw\"; no transfer decoding",
    CURLHELP_HTTP},