Browser: Process Max-Age cookie attribute

Note: the default expiry time should be the "the latest representable
date". However, DateTime::from_timestamp(NumericLimits<time_t>::max())
isn't feasible due to the for-loops in LibC's time_to_tm. So instead,
this just sets the date to the maxium year.

Userland/Applications/Browser/CookieJar.h

@@ -30,12 +30,14 @@
 #include <AK/Optional.h>
 #include <AK/String.h>
 #include <AK/Vector.h>
+#include <LibCore/DateTime.h>
 
 namespace Browser {
 
 struct Cookie {
     String name;
     String value;
+    Core::DateTime expiry_time {};
 };
 
 class CookieJar {