cmd/k8s-operator, net/netutil: support 4via6 in egress proxy and connector (#19863)

Add support for configuring egress to destinations reachable via 4via6
subnet routes. This change affects standalone egress proxy only- egress
ProxyGroup needs IPv6 support before being able to support 4via6. Egress may
be configured using either the synthesized 4via6 address or the MagicDNS
name (in the form
<IPv4-address-with-hyphens-instead-of-dots>-via-<siteid>[.*]).

Also update the Connector to validate and advertise 4via6 subnet routes.
Export net/netutil.ValidateViaPrefix so it can be reused by the Connector
validation logic.

Updates #19334

Signed-off-by: Becky Pauley <becky@tailscale.com>

cmd/k8s-operator/connector.go

@@ -29,6 +29,8 @@ import (
 	tsoperator "tailscale.com/k8s-operator"
 	tsapi "tailscale.com/k8s-operator/apis/v1alpha1"
 	"tailscale.com/kube/kubetypes"
+	"tailscale.com/net/netutil"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/tstime"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/set"
@@ -356,6 +358,11 @@ func validateRoutes(routes tsapi.Routes) error {
 		if pfx.Masked() != pfx {
 			errs = append(errs, fmt.Errorf("route %s has non-address bits set; expected %s", pfx, pfx.Masked()))
 		}
+		if tsaddr.IsViaPrefix(pfx) {
+			if err := netutil.ValidateViaPrefix(pfx); err != nil {
+				errs = append(errs, err)
+			}
+		}
 	}
 	return errors.Join(errs...)
 }