Move from hardcoded extensions to using extensions.json

[jlebon]

In coreos/coreos-assembler#2028, we now include an extensions.json in the m-o-c which details extensions available in the embedded yumrepo, as well as the corresponding RPMs, e.g.:

{
  "extensions": {
    "kernel-devel": {
      "packages": [
        "kernel-devel-4.18.0-240.15.1.el8_3",
        "kernel-headers-4.18.0-240.15.1.el8_3"
      ],
      "match-base-evr": "kernel",
      "kind": "os-extension"
    },
    "kernel": {
      "packages": [
        "kernel-4.18.0-240.15.1.el8_3",
        "kernel-core-4.18.0-240.15.1.el8_3",
        "kernel-modules-4.18.0-240.15.1.el8_3",
        "kernel-modules-extra-4.18.0-240.15.1.el8_3"
      ],
      "match-base-evr": "kernel",
      "kind": "development"
    },
    "kernel-rt": {
      "packages": [
        "kernel-rt-core",
        "kernel-rt-kvm",
        "kernel-rt-modules",
        "kernel-rt-modules-extra",
        "kernel-rt-devel"
      ],
      "architectures": [
        "x86_64"
      ],
      "kind": "os-extension"
    },
    "usbguard": {
      "packages": [
        "usbguard"
      ],
      "kind": "os-extension"
    }
  },
  "repos": [
    "rhel-8-nfv"
  ]
}

(The MCO should only look at extensions with kind: os-extension; the development ones are not meant for layering.)

Rather than hardcoding the list of supported extensions, the MCO could now read them from this JSON file and feed the package list to rpm-ostree.

For validation, there is also a com.coreos.os-extensions label on the m-o-c which lists the available extensions (see e.g. coreos/coreos-assembler#2028 (comment)).

[cgwalters]

I think part of the idea behind the hardcoded list in the MCO is that because it can be part of the CRD it can be validated at oc create time.

But that said probably simplest for now is to have a script that machine edits the CRD openapi validation from the current RHCOS build - we aren't going to need to run that very often.

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