Allow pipeline results to use custom task results

[GregDritschler]

Fixes #3595

This PR enables pipeline results to refer to pipeline tasks that run custom tasks and produce results.

Changes

Adds support to resolve pipeline result references to pipeline tasks that run custom tasks and produce results.

/kind feature

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Release Notes

Pipeline results now can refer to pipeline tasks that run custom tasks and produce results.

[GregDritschler]

@sbwsg fyi

[imjasonh]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ImJasonH, sbwsg

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