[bug#74819] [PATCH] services: elogind: Support Hook Directories

[45mg]

Here's how I tested these changes if anyone is interested:

- Add the files included here into a `testdir` directory
- Compile
  guix shell -D guix --pure -- make
- Test in a container (no loginctl sessions, but will catch obvious errors)
  sudo $(./pre-inst-env guix system container path/to/testdir/test-config.scm)
- Test in a VM (need --full-boot for the ability to suspend, etc)
  $(./pre-inst-env guix system vm --full-boot path/to/testdir/test-config.scm)

test-config.scm
Description: Binary data

test-sleep-script
Description: Binary data