cookie_remap: set_sendto_headers

[bneradt]

This adds a new set_sendto_headers configuration option that allows setting arbitrary HTTP headers (including the Host header) on requests to sendto destinations when cookie rules match. Header values support dynamic substitution of regex capture groups ($1, $2, etc.) and path variables ($path, $cr_req_url, etc.). When the Host header is set via this option, pristine_host_hdr is automatically disabled for that transaction.