feat(gen2-migration): implement decommission command with optimized stateful resource validation

[sai-ray]

Description of changes

This PR implements stateful resource validation for the decommission command in Gen2 migration. In the decommission command's validation section (decommission.ts), we scan CloudFormation changesets to detect stateful resources (DynamoDB tables, S3 buckets, Cognito user pools, etc.) scheduled for deletion and prevent accidental data loss by blocking the operation.

This PR also optimizes stateful resource validation (_validations.ts) by implementing rate-limited concurrent API calls using Bottleneck to prevent CloudFormation throttling during nested stack scanning. UX improvements include spinner feedback with category-specific progress messages and enhanced error handling with retry logic.

Issue #, if available

Description of how you validated changes

• Updated unit tests (_validations.test.ts) for stateful resource validation.
• Manual verification against real Amplify Gen1 projects with nested stacks.
• All tests pass successfully with yarn test.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

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