Useful when debugging issues (e.g. to see the full routing table), and
easier to refer to the output via a browser than trying to read it from
the logs generated by `bugreport --diagnose`.
Behind a canDebug() check, similar to the /magicsock and /interfaces
endpoints.
Updates #7184
Signed-off-by: Mihai Parparita <mihai@tailscale.com>