MINOR: proxy/http-ana: Add support of extra attributes for the cookie directive

MINOR: proxy/http-ana: Add support of extra attributes for the cookie directive

It is now possible to insert any attribute when a cookie is inserted by
HAProxy. Any value may be set, no check is performed except the syntax validity
(CTRL chars and ';' are forbidden). For instance, it may be used to add the
SameSite attribute:

    cookie SRV insert attr "SameSite=Strict"

The attr option may be repeated to add several attributes.

This patch should fix the issue #361.