fix(appsync-modelgen-plugin): map identityPool provider to IAM in generated Dart code

[jvh-aws]

Description of changes

fix(appsync-modelgen-plugin): map identityPool provider to IAM in generated Dart code

Codegen Paramaters Changed or Added

Issue #, if available

aws-amplify/amplify-flutter#6971

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• E2E test run linked
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• Breaking changes to existing customers are released behind a feature flag or major version update
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified
• Changes are tested on windows. Some Node functions (such as path) behave differently on windows.
• Changes adhere to the GraphQL Spec and supports the GraphQL types type, input, enum, interface, union and scalar types.

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