feat(migrate-template-gen): use nested file protocol operator using @

[abhi7cr]

Description of changes

Use AWS CLI new @= operator to support nested file protocols needed for stack refactor operations. This will remove the need for uploading the templates to S3 bucket.

AWS CLI >= 2.20 (https://raw.githubusercontent.com/aws/aws-cli/v2/CHANGELOG.rst)

• feature:shorthand: Adds support to shorthand syntax for loading parameters from files via the @= assignment operator.

Description of how you validated changes

Local testing using generate-templates command and running stack refactor operations from README file.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

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