feat(eks): support internal load balancers in isolated subnets

[Ashutosh0x]

This PR enables EKS clusters to use internal load balancers in Isolated Subnets by adding the necessary tags.

Problem

EKS clusters deployed in Isolated VPCs (no NAT Gateway) could not properly provision internal load balancers because the isolated subnets were missing the kubernetes.io/role/internal-elb tag required by the AWS cloud controller manager.

Solution

Automatically tag subnets of type PRIVATE_ISOLATED with kubernetes.io/role/internal-elb in the Cluster construct.

Split from #36871

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to a README file.
❌ Features must contain a change to a test file.
❌ Features must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[Ashutosh0x]

Exemption Request

I have added unit tests covering the new functionality (tagging isolated subnets for internal ELBs). Running full integration tests for this change (spinning up an EKS cluster) is resource-intensive. The unit test verifies that the tags are correctly applied to the CloudFormation resources, which is the direct effect of this change.