net/haproxy: support for cookie-based persistence

[fraenki]

This addition makes it pretty easy to configure cookie-based persistence. A new setting was added to the "Edit Backend Pool" configuration page:

When enabling cookie-based persistence, the following HAProxy options will be added or changed:

backend example_backend
     # stickiness
     stick-table type ip size 50k expire 30m
-    stick on src
+    cookie SRVCOOKIE prefix
     # tuning options
     timeout connect 30s
     timeout server 30s
-    server websrv1 10.0.0.1:80 check inter 1000 
+    server websrv1 10.0.0.1:80 check inter 1000  cookie 5aff65dd69e5e163736656

frontend example_frontend
     default_backend example_backend
+    # remove quotes from persistence cookie
+    http-request replace-header Cookie '^(.*?; )?(SRVCOOKIE=)"([^;"]*)"(;.*)?$' \1\2\3\4

The cookie value is derived from each server's unique ID. This value is automatically generated by OPNsense and it is guaranteed to never change, unless the server is removed from the GUI.

When using cookie-based persistence, the Balancing Algorithm should be changed from default "Source-IP Hash" to "Round Robin".