Capture remote stepAction source in SLSA resolvedDependencies

[ngelman1]

This PR aims to close some gaps before advancing to reslove issue #1160

Changes

When a TaskRun step uses a remotely resolved stepAction, Pipelines records its origin in StepState.Provenance.RefSource. This change wires that data into the SLSA provenance resolvedDependencies as entries named "stepAction/{stepName}", enabling full traceability of step definitions.

Currently, Chains captures the origin of remote Tasks and Pipelines in provenance, but ignores the origin of remote StepActions. This means the SLSA provenance has a traceability gap -- a verifier cannot tell where a step's definition came from.

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