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>

util/linuxfw/linuxfw.go

@@ -53,6 +53,13 @@ const (
 	FirewallModeNfTables FirewallMode = "nftables"
 )
 
+type CGNATMode string
+
+const (
+	CGNATModeDrop   CGNATMode = "DROP"
+	CGNATModeReturn CGNATMode = "RETURN"
+)
+
 // The following bits are added to packet marks for Tailscale use.
 //
 // We tried to pick bits sufficiently out of the way that it's