MINOR: http_ext: implement rfc7239_{nn,np} converters

MINOR: http_ext: implement rfc7239_{nn,np} converters

"option forwarded" provides a convenient way to automatically insert
rfc7239 forwarded header to requests sent to servers.

On the other hand, manually crafting the header is quite complicated due
to specific formatting rules that must be followed as per rfc7239.
However, sometimes it may be necessary to craft the header manually, for
instance if it has to be conditional or based on parameters that "option
forwarded" doesn't provide. To ease this task, in this patch we implement
rfc7239_nn and rfc7239_np which are respectively meant to craft nodename:
nodeport values, specifically intended to manually build rfc7239 'for'
and 'by' header fields while ensuring rfc7239 compliancy.

Example:
  # build RFC-compliant 7239 header:
  http-request set-var-fmt(txn.forwarded) "for=\"%[ipv6(::1),rfc7239_nn]:%[str(8888),rfc7239_np]\";host=\"haproxy.org\";proto=http"
  # check RFC-compliancy:
  http-request set-var(txn.test) "var(txn.forwarded),debug(ok,stderr),rfc7239_is_valid,debug(ok,stderr)"
  #  stderr output:
  #    [debug] ok: type=str <for="[::1]:_8888";host="haproxy.org";proto=http>
  #    [debug] ok: type=bool <1>

See documentation for more info and examples.