MINOR: http_ext: add rfc7239_is_valid converter

MINOR: http_ext: add rfc7239_is_valid converter

Adding new http converter: rfc7239_is_valid.

Takes a string representing 7239 forwarded header single value as
input and returns bool:TRUE if header is RFC compliant and
bool:FALSE otherwise.

  Example:
    acl valid req.hdr(forwarded),rfc7239_is_valid
    #input: "for=127.0.0.1;proto=http"
    #  output: TRUE
    #input: "proto=custom"
    #  output: FALSE

Depends on:
  - "MINOR: http_ext: introduce http ext converters"