nixos/opensmtpd: add useACMEHosts option

[m1cr0man]

Motivation for this change

Closes #101389

This allows the user to configure any number of certificates
to be loaded into opensmtpd on startup. A new service was added
to restart opensmtpd when certificates are updated.

This is the first time we're using LoadCredential. I have not found a way
to support hot reload with this method yet. It copies the cert files to a
special folder on start, and doesn't repeat the process if the files change
or if the service is reloaded.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[stale]

I marked this as stale due to inactivity. → More info