Submit product capture provider image refs

[intel352]

Summary

• add required provider_image_ref to step.compute_product_capture
• add --provider-image-ref to the product-capture submit CLI
• include the digest-pinned image_ref in provider workloads submitted to workflow-compute
• bump workflow-compute to the merged provider image_ref protocol

Verification

• GOPRIVATE=github.com/GoCodeAlone/* GOWORK=off go test ./... -count=1