[ECS] [request]: Task placement driven by another AWS resource's current AZ (e.g. RDS Multi-AZ primary)

**Tell us about your request**
Within a `placement_constraint` block, allow referencing attributes from another resource — examples would be the primary Availability Zone of a database instance.

**Which service(s) is this request for?**
ECS, ECS Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We run a DB-monitoring agent as an ECS service and want it colocated with our RDS Multi-AZ primary to avoid cross-AZ query cost. The primary's AZ can change on failover and the placement_constraint attribute can not track that. This problem exists with a number of AWS resources with a movable primary (ElastiCache, MSK, Redshift, etc).

**Are you currently working around this issue?**
We utilize a data lookup if/when we update the particular ECS service so we do have periods where the ECS task and target RDS primary are not co-located.

Start by reviewing the requested ECS and ECS Fargate behavior around placement_constraint and the examples involving RDS Multi-AZ, ElastiCache, MSK, and Redshift. The issue names no repository files, tests, or implementation entry points, so first determine where this roadmap request would map to an AWS service or API capability. Done means placement can follow a referenced resource's current primary Availability Zone after failover.

Written by the indexing model from the issue text.