MEDIUM: http: add new "capture" action for http-request

MEDIUM: http: add new "capture" action for http-request

This is only possible in frontends of course, but it will finally
make it possible to capture arbitrary http parts, including URL
parameters or parts of the message body.

It's worth noting that an ugly (char **) cast had to be done to
call sample_fetch_string() which is caused by a 5- or 6- levels
of inheritance of this type in the API. Here it's harmless since
the function uses it as a const, but this API madness must be
fixed, starting with the one or two rare functions that modify
the args and inflict this on each and every keyword parser.
(cherry picked from commit 484a4f38460593919a1c1d9a047a043198d69f45)