Feature request: Pure systemd service implementation

The use of activationScripts for decryption is very limiting for cases when the key is not static or is provided by an external source. It would be very useful to be able to be able to have agenix's decryption step done via a systemd service so dependencies can be added. In my usecase, I want a systemd service to attempt to mount a usb drive (containing the key), then after succeeding, trigger agenix. I dug around in the actual module, and [it seems](https://github.com/ryantm/agenix/blob/9edb1787864c4f59ae5074ad498b6272b3ec308d/modules/age.nix#L18-L22) there is a [systemd service that is created](https://github.com/ryantm/agenix/blob/9edb1787864c4f59ae5074ad498b6272b3ec308d/modules/age.nix#L283-L298), but only [systemd-sysusers](https://www.freedesktop.org/software/systemd/man/latest/systemd-sysusers.html) is enabled. I am not familiar with sysusers at all so I can't comment on it's functionality. But I think it could be useful to have this systemd service on non-sysuser setups. Thoughts?