[service] [request]: Allow me to choose the launch configuration or new Launch Template when creating a cluster

**Tell us about your request**
Please provide a dropdown menu to let me select a pre-existing launch configuration or launch template for ECS to use with my cluster.

**Which service(s) is this request for?**
This would be for ECS,

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently I have to create empty cluster and do lots of other things to make this work. Purpose is for me to easily have ECS use my own AMIs, launch configurations, auto-scaling groups, etc.

**Are you currently working around this issue?**
Have to create empty cluster and then do a lot of out-of-band config and custom userdata in AMI to associate with ECS cluster. This glues that AMI to a single cluster. Repeating this for different clusters is cumbersome.

Thank you for your time and a great service!

No files, tests, or implementation entry points are named in the issue. Start by clarifying the ECS cluster-creation flow and whether the request covers existing launch configurations, new launch templates, or both; done should include a defined selection experience and the required custom AMI, auto-scaling, and user-data behavior.

Written by the indexing model from the issue text.