Caddyfile support for On-Demand TLS permission modules

[mohammed90]

We're missing Caddyfile support for permission modules here. We should try to get that in for 2.8.0. (If anyone's reading this, PRs welcome if you want a little task)

Originally posted by @francislavoie in #6055 (comment)

[mholt]

Thanks, took care of it!