Allow 'oem' scheme for source url #2276

Created originally in Butane by @mpu-ssg: coreos/butane#634

Currently experimenting with Flatcar, I'm using butane to generate my ignition file.
Documentation suggests the use of oem:/// to retrieve files from /oem partition during ignition phase.

With the special oem:/// file URL the config can copy files from the OEM Partition to the root filesystem

Source: https://www.flatcar.org/docs/latest/installing/customizing-the-image/customize-the-image/#customizing-the-oem-partition

I haven't found any issue about this topic. Also it appears Ignition specification does not mention this, even the Flatcar one (https://www.flatcar.org/docs/latest/provisioning/ignition/specification/).