MINOR: http-ana: Add support for "set-cookie-fmt" option to redirect rules

MINOR: http-ana: Add support for "set-cookie-fmt" option to redirect rules

It is now possible to use a log-format string to define the "Set-Cookie"
header value of a response generated by a redirect rule. There is no special
check on the result format and it is not possible during the configuration
parsing. It is proably not a big deal because already existing "set-cookie"
and "clear-cookie" options don't perform any check.

Here is an example:

  http-request redirect location https://someurl.com/ set-cookie haproxy="%[var(txn.var)]"

This patch should fix the issue #1784.