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>

feature/tpm/tpm.go

@@ -40,6 +40,8 @@ var infoOnce = sync.OnceValue(info)
 func init() {
 	feature.Register("tpm")
 	feature.HookTPMAvailable.Set(tpmSupported)
+	feature.HookHardwareAttestationAvailable.Set(tpmSupported)
+
 	hostinfo.RegisterHostinfoNewHook(func(hi *tailcfg.Hostinfo) {
 		hi.TPM = infoOnce()
 	})