Add the src@optional scenario to bootc using [^cos9-src@optional.sh] file. Note that the "optional" container layer has been added in this PR.

The problem with the current implementation is that enp1s0 and enp2s0 interface names are hardcoded here and here. When booting a bootc-based images, we're using virtiofs to share container images between the hypervisor and the VM. This seems to affect the device naming.

See this Slack thread for more info.