[ECS] [request]: More granular status for Pending tasks

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**Tell us about your request**
More granular status around ECS task activities while in the pending state would be helpful to quickly diagnose potential problems. For example, beginning image download, ending image download.

**Which service(s) is this request for?**
ECS Fargate and likely EC2-based ECS.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently, it is difficult to diagnose why a task might be stuck in pending, and this can impede time-sensitive investigations into production failures. Presently, the Events log and the UI only shows "Pending" and provides little to no information on what is going on underneath the hood.

**Are you currently working around this issue?**
The only workaround we have is to wait for the Pending task to fail (which can be > hour) and then see what the failure is.

**Additional context**
There are a variety of potential solutions that could help. It could be added ECS events (though, this could flood the event log UI if there is no way to filter these). It could be a change to the UI for a task to show what it is currently engaged in doing (potentially avoiding the ECS event log entirely).

No files, tests, or implementation entry points are identified in this roadmap request. Start by reviewing how ECS Fargate and EC2-based ECS expose pending-task events and status, then define a concrete status or event surface; done would provide actionable visibility into stages such as image download.

Written by the indexing model from the issue text.