per-env/region artifact overrides on a Service

A Harness Service entity is a single object shared by every environment and infrastructure it’s deployed to, but its artifacts block is global to the service. We generate ECS services from a declarative config where the set of containers can differ per environment (a sidecar that runs in prd but not dev).

Today service overrides can set service variables per environment/infrastructure, but not artifact sources. Our workaround is to register the union of every environment’s containers on the service, which means each environment carries artifact entries it never references - those must still resolve and be pullable, forcing us to replicate images into every environment’s registry.

Ask: allow artifacts (primary sources and sidecars) to be overridden per region, the way service variables already can be.