cmd/{containerboot,k8s-operator}: add 4via6 support in singleton egress (#19983)

Add support for configuring egress to destinations reachable via 4via6
subnet routes, using either the synthesized 4via6 address or the MagicDNS
name (in the form <IPv4-with-hyphens>-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.

This change only affects standalone egress proxies — ProxyGroup egress
requires IPv6 support before it can use 4via6.

Updates #19334

Change-Id: I6faecd6eb61ab55fc0cd97fe417af6b6a12fe7fc

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

net/netutil/routes.go

@@ -17,7 +17,7 @@ import (
 // It verifies that the prefix is in the Tailscale via range, has a prefix
 // length between /96 and /128, and that the embedded site ID is in the
 // range 0–65535.
-func validateViaPrefix(ipp netip.Prefix) error {
+func ValidateViaPrefix(ipp netip.Prefix) error {
 	if !tsaddr.IsViaPrefix(ipp) {
 		return fmt.Errorf("%v is not a 4-in-6 prefix", ipp)
 	}
@@ -55,7 +55,7 @@ func CalcAdvertiseRoutes(advertiseRoutes string, advertiseDefaultRoute bool) ([]
 				return nil, fmt.Errorf("%s has non-address bits set; expected %s", ipp, ipp.Masked())
 			}
 			if tsaddr.IsViaPrefix(ipp) {
-				if err := validateViaPrefix(ipp); err != nil {
+				if err := ValidateViaPrefix(ipp); err != nil {
 					return nil, err
 				}
 			}