If an optional `hwaddrs` URL parameter is present, add network interface
hardware addresses to the posture identity response.
Just like with serial numbers, this requires client opt-in via MDM or
`tailscale set --posture-checking=true`
(https://tailscale.com/kb/1326/device-identity)
Updates tailscale/corp#21371
Signed-off-by: Anton Tolchanov <anton@tailscale.com>
