Caddy can now obtain certificates when behind load balancers and/or in
fleet/cluster configurations, without needing any extra configuration.
The only requirement is sharing the same $CADDYPATH/acme folder.
This works with the HTTP challenge, whereas before the DNS challenge
was required. This commit allows one Caddy instance to initiate the
HTTP challenge and another to complete it.

When sharing that folder, certificate management is synchronized and
coordinated, without the Caddy instances needing to know about each
other. No load balancer reconfiguration should be required, either.

Currently, this is only supported when using FileStorage for TLS
storage (which is ~99.999% of users).