This watches the provided path for a JSON encoded ipn.ServeConfig.
Everytime the file changes, or the nodes FQDN changes it reapplies
the ServeConfig.
At boot time, it nils out any previous ServeConfig just like tsnet does.
As the ServeConfig requires pre-existing knowledge of the nodes FQDN to do
SNI matching, it introduces a special `${TS_CERT_DOMAIN}` value in the JSON
file which is replaced with the known CertDomain before it is applied.
Updates #502
Updates #7895
Signed-off-by: Maisem Ali <maisem@tailscale.com>
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