healthcheck: add healthcheck to shutdown if cert is expired (#4792)

In certain container set ups, it's useful to optionally have lnd just shutdown if it detects that its certs are expired, as assuming there's a hypervisor to restart the container/pod, then upon restart, lnd will have fully up to date certs.

lncfg/healthcheck.go

@@ -26,6 +26,8 @@ type HealthCheckConfig struct {
 	ChainCheck *CheckConfig `group:"chainbackend" namespace:"chainbackend"`
 
 	DiskCheck *DiskCheckConfig `group:"diskspace" namespace:"diskspace"`
+
+	TLSCheck *CheckConfig `group:"tls" namespace:"tls"`
 }
 
 // Validate checks the values configured for our health checks.
@@ -38,6 +40,10 @@ func (h *HealthCheckConfig) Validate() error {
 		return err
 	}
 
+	if err := h.TLSCheck.validate("tls"); err != nil {
+		return err
+	}
+
 	if h.DiskCheck.RequiredRemaining < 0 ||
 		h.DiskCheck.RequiredRemaining >= 1 {