posture: add HealthTracker for serial number retrieval (#19181)

Device posture checking can fail while enabled if tailscaled does not
have access to smbios. Previously, this was only observable by looking
in the tailscaled logs.

Fixes tailscale/corp#39314

Signed-off-by: Evan Lowry <evan@tailscale.com>

posture/serialnumber_stub.go

@@ -12,6 +12,7 @@ package posture
 
 import (
 	"errors"
+	"fmt"
 
 	"tailscale.com/types/logger"
 	"tailscale.com/util/syspolicy/policyclient"
@@ -19,5 +20,5 @@ import (
 
 // GetSerialNumber returns client machine serial number(s).
 func GetSerialNumbers(polc policyclient.Client, _ logger.Logf) ([]string, error) {
-	return nil, errors.New("not implemented")
+	return nil, fmt.Errorf("not implemented: %w", errors.ErrUnsupported)
 }