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.

sample-lnd.conf

@@ -846,6 +846,22 @@ litecoin.node=ltcd
 ; value must be >= 1m.
 ; healthcheck.diskspace.interval=6h
 
+; The number of times we should attempt to check for certificate expiration before
+; gracefully shutting down. Set this value to 0 to disable this health check.
+; healthcheck.tls.attempts=2
+
+; The amount of time we allow a query for certificate expiration to take
+; before we fail the attempt. This value must be >= 1s.
+; healthcheck.tls.timeout=5s
+
+; The amount of time we should backoff between failed attempts to query
+; certificate expiration. This value must be >= 1s.
+; healthcheck.tls.backoff=1m
+
+; The amount of time we should wait between certificate expiration health checks.
+; This value must be >= 1m.
+; healthcheck.tls.interval=1m
+
 [signrpc]
 
 ; Path to the signer macaroon.