[ECS] [networking]: allow overriding port mappings in runTask

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**Tell us about your request**
Right now, in containerOverrides in runTask, you can override everything you could override in a docker run command besides runtime and port mappings -- I'd like to be able to override port mappings.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I'm running containers that need specific ports forwarded to function -- in order to run multiple containers per instance, I need to be able to have specialized port forwarding set up for each, which means dynamic port forwarding. However, as far as I can tell, with dynamic port forwarding (with host port set to 0), there's no easy way to see which reserved ephemeral port goes to which container port -- which means the best option seems to be to just set my own port mappings at run task time by directly checking reserved/available ports on my ec2 instances?

**Are you currently working around this issue?**
Right now I'm programmatically making a new revision of my task def for every new portmapping config. This feels like an insane antipattern.

**Additional context**
Anything else we should know?

**Attachments**
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The issue concerns AWS ECS runTask containerOverrides and port mappings, but it does not name source files, tests, or a code entry point. Start by determining whether this roadmap request maps to an available implementation repository; done would require supporting port-mapping overrides without creating a new task-definition revision and verifying the behavior.

Written by the indexing model from the issue text.