Allow the ECS Blue Green Traffic Shift step in Standalone mode to operate on the same listener rule ARN that was referenced in the ECS Blue Green Create Service step. Today this is blocked with:

▎ Invalid request: Standalone traffic shifting can't be performed on the rule specified in Blue Green Create Service step

From the customer's perspective, the rule referenced in the Create Service step is just another rule pointing at the blue/green target groups. It's unclear why that particular rule is singled out as ineligible for Standalone shifting, when other rules pointing at the same target groups are allowed. The enforcement appears to assume a 1:1 Create-Service-to-rule relationship that doesn't match real-world multi-rule services.

This would simplify our (and other customers using ECS deployments) steps, as right now we need a single inherit step to account for the listener rule ARN in "ECS Blue Green Create Service", followed by multiple standalone steps to account for other listener rules.