fix(cloudformation-diff): move aws-sdk to dependency for cfn-diff to get CFN types resolved in exports (#28768)

Issue still persists after #28680. 
```
node_modules/@aws-cdk/cloudformation-diff/lib/diff-template.d.ts:1:37 - error TS2307: Cannot find module 'aws-sdk' or its corresponding type declarations.

1 import type { CloudFormation } from 'aws-sdk';
                                      ~~~~~~~~~


Found 1 error in node_modules/@aws-cdk/cloudformation-diff/lib/diff-template.d.ts:1
```

Types are still required as a direct dependency in package.json.

Closes #28679 

----

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

Author: frankpengau

packages/@aws-cdk/cloudformation-diff/package.json

@@ -25,6 +25,7 @@
   "dependencies": {
     "@aws-cdk/aws-service-spec": "^0.0.53",
     "@aws-cdk/service-spec-types": "^0.0.53",
+    "aws-sdk": "2.1576.0",
     "chalk": "^4",
     "diff": "^5.2.0",
     "fast-deep-equal": "^3.1.3",
@@ -38,7 +39,6 @@
     "@types/string-width": "^4.0.1",
     "fast-check": "^3.16.0",
     "jest": "^29.7.0",
-    "aws-sdk": "2.1576.0",
     "ts-jest": "^29.1.2"
   },
   "repository": {