nixos/networkd: allow configuring AckFilter for CAKE qdisc

The CAKE section for systemd.network units allows configuring whether or
not redundant ACKs should be dropped. This option corresponds to the
respective tc-cake(8) params "ack-filter", "ack-filter-aggressive" or
"no-ack-filter".

Add support for these values in the `cakeConfig` module so that users
can configure it.

nixos/modules/system/boot/networkd.nix

···
           "FirewallMark"
           "Wash"
           "SplitGSO"
+          "AckFilter"
         ])
         (assertValueOneOf "AutoRateIngress" boolValues)
         (assertInt "OverheadBytes")
···
         (assertRange "FirewallMark" 1 4294967295)
         (assertValueOneOf "Wash" boolValues)
         (assertValueOneOf "SplitGSO" boolValues)
+        (assertValueOneOf "AckFilter" (boolValues ++ ["aggressive"]))
       ];
 
       sectionControlledDelay = checkUnitConfig "ControlledDelay" [