util/linuxfw,wgengine/router: allow incoming CGNAT range traffic with nodeattr

Clients with the newly added node attribute
`"disable-linux-cgnat-drop-rule"` will not automatically drop inbound
traffic on non-Tailscale network interfaces with the source IP in the
CGNAT IP range. This is an initial proof-of-concept for enabling
connectivity with off-Tailnet CGNAT endpoints.

Fixes tailscale/corp#36270.

Signed-off-by: Naman Sood <mail@nsood.in>

wgengine/router/router_test.go

@@ -15,7 +15,7 @@ func TestConfigEqual(t *testing.T) {
 	testedFields := []string{
 		"LocalAddrs", "Routes", "LocalRoutes", "NewMTU",
 		"SubnetRoutes", "SNATSubnetRoutes", "StatefulFiltering",
-		"NetfilterMode", "NetfilterKind",
+		"NetfilterMode", "NetfilterKind", "RemoveCGNATDropRule",
 	}
 	configType := reflect.TypeFor[Config]()
 	configFields := []string{}