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>
35a1a413f9