chore(portforward): move vpn gateway obtention within port forwarding service

internal/portforward/interfaces.go

@@ -1,6 +1,9 @@
 package portforward
 
-import "context"
+import (
+	"context"
+	"net/netip"
+)
 
 type Service interface {
 	Start(ctx context.Context) (runError <-chan error, err error)
@@ -8,6 +11,10 @@ type Service interface {
 	GetPortForwarded() (port uint16)
 }
 
+type Routing interface {
+	VPNLocalGatewayIP(vpnInterface string) (gateway netip.Addr, err error)
+}
+
 type PortAllower interface {
 	SetAllowedPort(ctx context.Context, port uint16, intf string) (err error)
 	RemoveAllowedPort(ctx context.Context, port uint16) (err error)