V1: add conversion for PipelineRunSpec.PipelineRef.Bundle

[JeromeJu]

Changes

This commit adds support for PipelineRef.Bundle when converting between
v1beta1 and v1 versions of PipelineRef. This allows us to release v1
PipelineRuns in a backwards compatible way by ensuring that v1beta1
PipelineRuns with Bundle converted to v1 pipelineRuns.

part of #4987
/kind misc

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Release Notes

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lbernick

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[abayer]

/lgtm