Add current iteration item to the PipelineLoopPipelineRunStatus

[wzhanw]

Which issue is resolved by this Pull Request:
Resolves #

Description of your changes:
The current PipelineLoopPipelineRunStatus only has the Iteration which is an integer value, sometimes it is meaningless for the user. We should have IterationItem in the status as well.
Environment tested:
Unit test passed.

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

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[yhwang]

/lgtm

Thanks for also fix the cache pkg

[Tomcli]

Thanks @wzhanw, e2e test passed for this PR
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Tomcli, wzhanw, yhwang

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