Suggestion: add sugar for systemd-confext

Created originally in Butane by @jbtrystram: https://github.com/coreos/butane/issues/700

Systemd-confext bring a nice way to do out of band configuration management.
In it's simplest form, it's a directory under var/lib/confexts/<confext-name>. All content under that is overlayed on top of /etc.

One of the quirks is that it makes /etc read-only, but this is not a concern for ignition, since systemd-confext isn't active yet in the initramfs.

I think it would be nice to support fetching confexts fron a few sources, like HTTP and GIT, e.g. :

confext:
  name: quadlets
  source: https://github.com/user/repo
  enabled: true

This provide a nice way to update nodes day 2, but in a more declarative way. Because re-spin a whole FCOS node to add a new quadlet file through ignition can feel a bit overkill, especially on bare-metal.

see https://github.com/coreos/fedora-coreos-tracker/issues/2108
And https://www.man7.org/linux/man-pages/man8/systemd-confext.8.html

Start with the linked Butane issue #700, the Fedora CoreOS tracker issue #2108, and the systemd-confext manual page to understand the desired configuration model and source behavior. Define the supported HTTP and Git source semantics, including the example name and enabled fields, so that declarative confext deployment is complete and unambiguous.