Reload origincert without a restart

## Feature request

Add the ability to reload the origincert file without restarting the daemon, ideally via SIGHUP signal.

## Motivation

We want to rotate origin origin certs frequently. This requires editing the origincert file and then restarting cloudflared. This causes all in-flight requests from our users to be dropped.

#### More background
We use cloudflared for Argo Tunnels on our hosts. We *do not use* `cloudflared login`, but rather provision the argo tokens and origin certs separately and copy them to the hosts. We do this because it is more secure then the default which issues a wildcard cert.