feat(ecs): new L2 construct for ManagedInstances CapacityProvider (#35648)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

<!--What is the bug or use case behind this change?-->

ECS is launching new Capacity Provider called Managed Instances similar
to currently available [AutoScalingGroup Capacity
Provider](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ecs.AsgCapacityProvider.html)

### Description of changes

<!--
What code changes did you make? 
Why do these changes address the issue?
What alternatives did you consider and reject?
What design decisions have you made?
-->

Created a new L2 construct for ManagedInstances Capacity Provider.

AWS ECS Managed Instances is a new capability being launched, that
allows customers to specify desired Amazon EC2 instance types for their
serverless workloads. It enables them to run workloads that require
specific compute capabilities, such as GPU, accelerated compute, CPU
instruction sets, or large number of vCPUs, while retaining the
serverless benefits of fully managed compute infrastructure and
automatic security patching. AWS ECS Managed Instances leverages Amazon
EC2 Managed Instances to deliver these compute capabilities, with
seamless access to EC2 features like Reserved Instances (RIs) and
On-Demand Capacity Reservations (ODCRs).

Public API doc: 

[Documentation](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ManagedInstances.html)
[AWS News
Blog](https://aws.amazon.com/blogs/aws/announcing-amazon-ecs-managed-instances-for-containerized-applications/)
[What’s New
Post](https://aws.amazon.com/about-aws/whats-new/2025/09/amazon-ecs-managed-instances/)
[Product Detail Page](https://aws.amazon.com/ecs/managed-instances/)
[Pricing Page](https://aws.amazon.com/ecs/managed-instances/pricing/)
[FAQs](https://aws.amazon.com/ecs/managed-instances/faqs/)
### Describe any new or updated permissions being added

<!-- What new or updated IAM permissions are needed to support the
changes being introduced? -->

This feature introduced a new role called `infrastructureRole`.

### Description of how you validated changes

<!-- Have you added any unit tests and/or integration tests? Did you
test by hand? -->

- Added an integration test
- Added unit tests

### Checklist
- [X] My code adheres to the [CONTRIBUTING
GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and
[DESIGN
GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

Author: kg-aws