This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.
The PEM loader allows you to embed PEM files (certificates and keys)
directly into your config, rather than requiring them to be stored on
potentially insecure storage, which adds attack vectors. This is useful
in automated settings where sensitive key material is stored only in
memory.
Note that if the config is persisted to disk, that added benefit may go
away, but there will still be the benefit of having lesser dependence on
external files.
