fix(cloudformation-diff): Fix aws-sdk dependency issue (#28680)

Missing dependency when exporting `@aws-cdk/cloudformation-diff` in custom AWS Construct Library. 

Closes #28679

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Author: frankpengau

packages/@aws-cdk/cloudformation-diff/lib/diff-template.ts

@@ -1,7 +1,6 @@
 // The SDK is only used to reference `DescribeChangeSetOutput`, so the SDK is added as a devDependency.
 // The SDK should not make network calls here
-// eslint-disable-next-line import/no-extraneous-dependencies
-import { CloudFormation } from 'aws-sdk';
+import type { CloudFormation } from 'aws-sdk';
 import * as impl from './diff';
 import * as types from './diff/types';
 import { deepEqual, diffKeyedEntities, unionOf } from './diff/util';