(aws-ecs): scaleOnRequestCount should take IApplicationTargetGroup

### Description

`aws_ecs.ScalableTaskCount.scaleOnRequestCount` requires the targetGroup to be `ApplicationTargetGroup` which prevents it from working with load balancers referenced via `aws_elasticloadbalancingv2.ApplicationTargetGroup.fromTargetGroupAttributes`

https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-ecs.RequestCountScalingProps.html

1.

### Use Case

I'd like to use request count autoscaling with a load balancer that was created outside the current cdk application. I'm referencing this load balancer via `ApplicationTargetGroup.fromTargetGroupAttributes`

### Proposed Solution

`IApplicationTargetGroup` could have the required properties added to it (firstLoadBalancerFullName and targetGroupFullName)

### Other information

_No response_

### Acknowledge

- [X] I may be able to implement this feature request
- [X] This feature might incur a breaking change

Start with aws_ecs.ScalableTaskCount.scaleOnRequestCount and RequestCountScalingProps, then inspect ApplicationTargetGroup.fromTargetGroupAttributes and the IApplicationTargetGroup interface. Determine the required target-group properties and verify that imported application target groups can be used for request-count autoscaling without breaking existing behavior.

Written by the indexing model from the issue text.