fix: un-require required input fields with @default

[p5quared]

Description of changes

Un-require required input fields that have @default applied. Also update the corresponding snapshot to document this behavior.

Prior to this change, a copy of the input object was being modified but ctx.output was not being updated with the new input.

CDK / CloudFormation Parameters Changed

n/a

Issue #, if available

https://github.com/aws-amplify/amplify-codegen/issues/390

Description of how you validated changes

Fixed a test snapshot and passed it.

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] 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.