Extend Persist with AttestationKey to record a hardware-backed
attestation key for the node's identity.
Add a flag to tailscaled to allow users to control the use of
hardware-backed keys to bind node identity to individual machines.
Updates tailscale/corp#31269
Change-Id: Idcf40d730a448d85f07f1bebf387f086d4c58be3
Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>