✨ download BM images via oci-registry.

[guettli]

What this PR does / why we need it:

Up to now, bare-metal machine images could only be downloaded from http/https URLs.

This PR make it possible to download the machine images from oci-registries.

You can use a tool like oras to upload the machine image in tgz format to an oci registry.

[janiskemper]

does it make sense to have the shell script in the Go code? Shouldn't it be rather in hack/?

[guettli]

does it make sense to have the shell script in the Go code? Shouldn't it be rather in hack/?

The shell script does not get executed like during build/compile time like the other scripts.

The shell scripts get copied from the controller intpkg/services/baremetal/client/ssh/download-from-oci.sho the rescue-system. I added a comment at the top.

I think hack/ is no good place for this script.

I could copy the script into the Go code like (instead of using embed):

var script=`#!/bin/bash
....

@janiskemper what do you think? I can copy it to hack/ if you prefer that.

[janiskemper]

I think I prefer having it hard-coded in the Go code!

[guettli]

I think I prefer having it hard-coded in the Go code!

ok, I put the shell script into the Go code.