fix: add default ref for unauthrole for imported auth

[AaronZyLee]

Description of changes

Add the default unauth role name reference for import auth since it is required for child stacks.

Issue #, if available

Fix https://github.com/aws-amplify/amplify-category-api/issues/677

Description of how you validated changes

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

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

[codecov-commenter]

Codecov Report

Merging #10848 (035ae74) into dev (2b17187) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev   #10848   +/-   ##
=======================================
  Coverage   47.43%   47.43%           
=======================================
  Files         673      673           
  Lines       33274    33274           
  Branches     6724     6725    +1     
=======================================
  Hits        15782    15782           
  Misses      15803    15803           
  Partials     1689     1689           

Impacted Files	Coverage Δ
...y-provider-awscloudformation/src/push-resources.ts	18.62% <0.00%> (ø)

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[danielleadams]

@AaronZyLee are you still working on this?