Consume OCI images for machine image

allow podman machine to extract its disk image from an oci registry or
oci-dir locally.  for now, the image must be relatively inflexible. it
must have 1 layer.  the layer must possess one image. so a dockerfile
like:

FROM scratch
COPY ./myimage.xz /myimage.xz

when using an oci dir, the directory structure must adhere to the
typical directory structure of a an oci image (with one layer).

── blobs
│   └── sha256
│       ├── 53735773573b3853bb1cae16dd21061beb416239ceb78d4ef1f2a0609f7e843b
│       ├── 80577866ec13c041693e17de61444b4696137623803c3d87f92e4f28a1f4e87b
│       └── af57637ac1ab12f833e3cfa886027cc9834a755a437d0e1cf48b5d4778af7a4e
├── index.json
└── oci-layout

in order to identify this new input, you must use a transport/schema to
differentiate from current podman machine init --image-path behavior. we
will support `oci-dir://` and `docker://` as transports.

when using the docker transport, you can only use an empty transport for
input.  for example, `podman machine init --image-path docker://`.  A
fully quailified image name will be supported in the next iteration.

the transport absent anything means, i want to pull the default fcos
image stored in a registry.  podman will determine its current version
and then look for its correlating manifest.  in this default use case,
it would look for:

quay.io/libpod/podman-machine-images:<version>

that manifest would then point to specific images that contain the
correct arch and provider disk image. i.e.

quay.io/libpod/podman-machine-images:4.6-qcow2

this PR does not enable something like
docker://quay.io/mycorp/myimage:latest yet.

names, addresses, andf schema/transports are all subject to change. the
plan is to keep this all undocumented until things firm up.

[NO NEW TESTS NEEDED]

Signed-off-by: Brent Baude <bbaude@redhat.com>

pkg/machine/config.go

@@ -585,6 +585,7 @@ func (dl Download) AcquireVMImage(imagePath string) (*VMFile, FCOSStream, error)
 		imageLocation *VMFile
 		fcosStream    FCOSStream
 	)
+
 	switch imagePath {
 	// TODO these need to be re-typed as FCOSStreams
 	case Testing.String(), Next.String(), Stable.String(), "":