quadlet: Add a network requirement on .image units

If a container unit starts on boot with a dependency on `default.target`
the image unit may start too soon, before network is ready. This cause
the unit to fail to pull the image.
- Add a dependency on `network-online.target` to make sure image pulls
don't fail.
See https://github.com/containers/podman/issues/21873

- Document the hardcoded dependency on `network-online.target` for images unit
and explain how it can be overriden if necessary.

- tests/e2e/quadlet: Add `assert-last-key-regex`

Required to test the `After=` override in [Unit] section
See https://github.com/containers/podman/pull/22057#issuecomment-2008959993

- quadlet/unitfile: add a prepenUnitLine method

Requirements on networks should be inserted at the top of the
section so the user can override them.

Signed-off-by: jbtrystram <jbtrystram@redhat.com>

test/e2e/quadlet/basic.image

@@ -1,4 +1,6 @@
 ## assert-podman-final-args localhost/imagename
+## assert-key-is "Unit" "After" "network-online.target"
+## assert-key-is "Unit" "Wants" "network-online.target"
 ## assert-key-is "Unit" "RequiresMountsFor" "%t/containers"
 ## assert-key-is "Service" "Type" "oneshot"
 ## assert-key-is "Service" "RemainAfterExit" "yes"