ipn/ipnlocal: add PeerAPI endpoint for doctor output

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>
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