Support systemd-credentials provider

[con-f-use]

Systemd-credentials are a way to securely store passwords on a machine. The machines TPM (or software stand-in) handles encryption while systemd handles access management on the machine.

The mechanism is already used by many NixOS modules, e.g. forgejo

[blaggacao]

Though it seems that the liked forgejo doesn't use LoadCredentialEncrypted which - my current understanding - was the the correct way