Allow loading secrets via external file

[Mistyttm]

I'm trying to write a module for NixOS that allows mautrix-discord to be used. However, I'm having issues with the fact that secrets are world readable. The option we've come up with is to substitute the secrets from a non-world readable file, but that still allows the secrets to end up world readable in both the config and registration files.

Ideally there would be an option to supply additional config or secret files to mautrix-discord to allow for non-world readable secrets in the registration and config files.