Implement OBS stage workflow

[xmudrii]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR implements OBS staging workflow via krel obs stage command as described in #3051. For more details about the command workflow, including a diagram, see the referenced issue.

In general, this workflow does the following:

• generate release version, package version, OBS project name
• check prerequisites such as environment variables and disk space
• run osc checkout to checkout the project locally
• run krel obs stage for each package
• run osc addremove to stage changes
• run osc commit to commit and push changes to OBS

Pushing changes to OBS triggers the package building process. Packages are not yet available to end users as this requires a release step that's going to be implemented in a follow up PR.

This workflow is realized as a dedicated step in the stage process, see changes to gcb/stage/cloudbuild.yaml for details. This workflow can also be called standalone using the submit option. The submit option is going to trigger a new Cloud Build job responsible only for building packages. This option is supposed to be used by other Kubernetes subprojects.

The scope of this PR is only kubeadm, kubectl, and kubelet. cri-tools and kubernetes-cni can be built using the standalone mode described above. We're yet going to figure out how we can automate those two packages.

This PR has some TODO notes that are non-blocking and will be resolved in follow up PRs.

This PR has been tested and logs are available here: https://console.cloud.google.com/cloud-build/builds/4fc2cb3b-81ee-4336-8834-1e8414aa43c0?project=kubernetes-release-test

It can also be tested by triggering a mock stage process (although packages are not going to be committed/pushed to OBS because it's mock stage):

export TOOL_ORG=xmudrii TOOL_REF=obs-stage && go run ./cmd/krel stage

Which issue(s) this PR fixes:

Fixes #3051

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Implement OBS staging workflow via `krel obs stage` command

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[xmudrii]

The PR is ready for review
/assign @saschagrunert @cpanato @puerco
cc @kubernetes/release-engineering

[xmudrii]

/test pull-release-integration-test-eks-canary

[xmudrii]

/test pull-release-verify

[xmudrii]

/assign

[xmudrii]

/assign @saschagrunert @cpanato @xmudrii

[saschagrunert]

Great work, just found a non-blocking nit with the license header. This can be fixed alter on.

[saschagrunert]

Suggested change

	Copyright 2020 The Kubernetes Authors.
	Copyright 2023 The Kubernetes Authors.

[xmudrii]

I’ll fix it in the PR for OBS release workflow.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: saschagrunert, xmudrii

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