types/persist: add AttestationKey (#17281)

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>

ipn/prefs.go

@@ -709,6 +709,7 @@ func NewPrefs() *Prefs {
 	// Provide default values for options which might be missing
 	// from the json data for any reason. The json can still
 	// override them to false.
+
 	p := &Prefs{
 		// ControlURL is explicitly not set to signal that
 		// it's not yet configured, which relaxes the CLI "up"