Support PipelineRun minimal embedded status

[abayer]

Starting in Pipeline v0.35.0, we support a new approach to embedding TaskRun and Run statuses in PipelineRun.Status, as described in TEP-0100. When the embedded-status feature flag is set to minimal, the PipelineRun.Status.TaskRuns and PipelineRun.Status.Runs fields aren't populated. Instead, a new field, PipelineRun.Status.ChildReferences is populated with a list of the API version, kind, name, and PipelineTask name for each TaskRun or Run in the PipelineRun, requiring anything trying to see the status of the children of a PipelineRun to fetch those statuses directly.

The CLI needs to be updated to support minimal embedded status in the next few months, since the default will be switching to minimal in Pipeline v0.44.0 and the feature flag will be going away in Pipeline v0.45.0.

This is a requirement for tektoncd/pipeline#4954

/kind feature
/kind tep