fix(eks): imported clusters can't deploy manifests

[pahud]

The imported eks clusters are having invalid service token and can't deploy new k8s manifests or helm charts. This PR fixes that issue.

• Update README and doc string. functionArn should be the custom resource provider's service token rather than the kubectl provider lambda arn. No breaking change in this PR.
• Add a new integration test to ensure the imported cluster can always create manifests and helm charts.

Closes #25835

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[gitpod-io]

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[pahud]

Hi @otaviomacedo please take a look again. Thanks.

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[aws-cdk-automation]

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 5a862f6
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[dancmeyers]

I'm afraid this doesn't fix the reported error. See #25938