Gen 1 migrations

[dpilch]

Description of changes

• Add transformer option enableGen2Migration.
  • Set dynamodb table data source removal policy to retain when true.
  • Output a mapping of model name to table name when true.
  • This will be exposed as a feature flag to gen 1 customers.

CDK / CloudFormation Parameters Changed

New behavior is not exposed through the construct because migration is not supported from construct to gen 2.

Issue #, if available

N/A

Description of how you validated changes

• Add transformer option enableGen2Migration.
  • Set dynamodb table data source removal policy to retain when true.
  • Output a mapping of model name to table name when true.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Any CDK or CloudFormation parameter changes are called out explicitly

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

[dpilch]

Closing in favor of #3084