Add VAT predicate for staging images if env var is passed

Background

To support using STAGING_BASE_IMAGE and getting through the VAT stage, we need to add a method for including the VAT predicate for images that have only been pushed to ironbank-staging.

To do this, we'll want to:

• Add an extra optional job at the end of the pipeline
• On any branch, allow "SOME_RELEVANT_VAR_NAME" to be manually passed to run this job
• If this var is passed, this job will
  • use the parent_vat_response.json and vat_response.json artifacts from the vat stage
  • combine the two json reports
    • this code already exists for the harbor job, so we'll want take the code from there and make it reusable for both jobs
  • publish this combined report as a predicate for the image in ironbank-staging

Acceptance Criteria

• A new optional job is created to publish the vat predicate
• An env var is checked to run this job
• Artifacts are reused from the VAT job
• This process is documented in ironbank-docs, or another place CHT can see it

Definition of Done

• Write or update any unit or integration tests
• Project pipeline runs successfully
• Solution is captured as code and/or documentation and merge requests have been submitted
• Code review completed and merge request approved/merged
• All Acceptance Criteria have been completed