Proposal: requires for ProwJob

[deedubs]

What would you like to be added:

I would like to be able to create a DAG of prow jobs that need to run in order, f.e.

presubmits:
  - name: build_container
    ....
  - name: deploy_preview
    requires: 
      - build_container

The prior art of this in use would be in CircleCI https://circleci.com/docs/2.0/workflows/#sequential-job-execution-example

Why is this needed:

It would allow simple workflows without having to reach for tekton which is a much larger commitment

[deedubs]

/area prow

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[k8s-ci-robot]

@fejta-bot: Closing this issue.

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