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>
BeckyPauleyandGitHub
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