MINOR: sample: Add strcmp sample converter

MINOR: sample: Add strcmp sample converter

This converter supplements the existing string matching by allowing
strings to be converted to a variable.

Example usage:

  http-request set-var(txn.host) hdr(host)
  # Check whether the client is attempting domain fronting.
  acl ssl_sni_http_host_match ssl_fc_sni,strcmp(txn.host) eq 0