RFE: modify `cosa buildfetch` to support OCP versions

[miabbott]

Feature Request

I'd like to be able to specify an OCP version when doing cosa buildfetch so that I can see a more accurate representation of changes when doing a new RHCOS build.

Desired Feature

Ultimately, I want to be able to accurately report the different RHSAs that are included in an RHCOS build when compared to the last z-stream release for an OCP version.

Example Usage

$ cosa buildfetch --ocp 4.10
Fetching update graph for OCP stable-4.10 stream...
Found latest OCP 4.10.z release: 4.10.14
Found RHCOS 410.84.202205131801-0
Updated builds/builds.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/meta.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/commitmeta.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/ostree-commit-object

Other Information

Could potentially provide another flag to select the OCP channel (eus, fast, stable)

$ cosa buildfetch --ocp 4.10 --channel fast
Fetching update graph for OCP fast-4.10 stream...
Found latest OCP 4.10.z release: 4.10.14
Found RHCOS 410.84.202205131801-0
Updated builds/builds.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/meta.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/commitmeta.json
Fetching: s3://art-rhcos-ci/releases/rhcos-4.10/410.84.202205131801-0/x86_64/ostree-commit-object

[cgwalters]

I'd personally prefer doing https://github.com/coreos/coreos-assembler/issues/2685 first, then the UX becomes clearer because as part of that, we explicitly create e.g. a distinct, public facing quay.io/openshift/rhel-coreos-86:4.12.3 that can be used to init the build.

(The mapping between channels and release images and a rhcos version still applies)