This PR adds the ability to pass in an --advertise-routes flag
to the sniproxy so that the sniproxy itself will also act as a
subnet router. This, alongside approving a Tailnet for extraRecords,
will allow 4via6, essentially pointing domains to a IPv6 address
that the sniproxy is advertising.
Updates #15372
Signed-off-by: Richard Castro <richard@tailscale.com>