add oci config

[mac641]

Description

References

• add oci pull to images metal-hammer#175
• add oci push metal-images#375
• implement image check for oci pull metal-api#635

[mac641]

@majst01 I added a way of parsing multiple oci configs using StringSlice. I think this is a little ugly, if you know a better way to solve this, please let me know.

Besides, how can I test pixiecore's grpccmd?

[mac641]

@majst01 As discussed, this the updated version of the oci pull config where a yaml file is used in order to pass over registry-urls and optional login info.

My implementation expects the file to look something like this:

oci-configs:
  - registry-url: oci://ghcr.io/metal-images/debian:12
    username: bla
    password: blub
  - registry-url: oci://ghcr.io/metal-images/debian:12
    username: ""
    password: ""
  - registry-url: oci://ghcr.io/metal-images/almalinux:9
    username: foo
    password: bar

Since we don't have any login info at this point, I have a couple of questions about this:

• How is grpccmd tested? How can I test it locally?
• What do I need to adjust in the mini-lab in order to have this file present?
• and - generally speaking - where does pixiecore get the infos for MetalConfig from, especially in the case of the mini-lab?