fix(integ-runner): fails with "no such file or directory, open 'node_…

…modules/@aws-cdk/integ-runner/lib/workers/db.json.gz'" (#27983)

Mark the `@aws-cdk/aws-service-spec` as external, because it cannot be bundled.

Changes an import from `cdk-assets` to be more specific, so we don't bundle the whole of `aws-sdk` anymore.
This reduced the worker entrypoint size from 70MB -> 1.5MB

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