kargo: cool down upstream releases with a holding stage

♠ Problem statement: wait before opening an upstream Helm chart upgrade PR, without putting a timer in the PR workflow. In other words: implement dependency cooldowns1 in Kargo. A Kargo Warehouse makes newly discovered Freight immediately available to a Stage that accepts it directly: Warehouse ──immediate──> open-upgrade-pr ──> PR Kargo already has requiredSoakTime, but only between Stages. It cannot delay a direct Warehouse-to-Stage promotion. So add a Stage whose promotion deliberately does nothing: apiVersion: kargo.akuity.io/v1alpha1 kind: Stage metadata: name: hold-upstream-release spec: requestedFreight: - origin: kind: Warehouse name: external-secrets-upstream sources: direct: true promotionTemplate: spec: steps: - uses: compose-output config: state: holding The PR Stage accepts Freight from that holding Stage and starts only after the cool-off period: spec: requestedFreight: - origin: kind: Warehouse name: external-secrets-upstream sources: stages: - hold-upstream-release…