MINOR: http_ext/7239: warn the user when fetch is not available

MINOR: http_ext/7239: warn the user when fetch is not available

http option forwarded (rfc7239) supports sample expressions when
configuring 'host', 'for' and 'by' parameters.

However, since we are in a http-backend-only context, right after http
header is processed, we have a limited resolution scope for the sample
expression provided by the user.

To prevent any confusion, a warning is emitted when parsing the option
if the user relies on a sample expression (more precisely a fetch) which
would yield unexpected results at runtime when processing the option.