How to ensure a field in a type is unique on recent amplify version

### Amplify CLI Version

12.3.0

### Question

I'm trying to define a unique field in my model. I've tried this guide: https://dev.to/callmekatootie/aws-appsync-with-amplify-how-to-ensure-a-field-in-a-type-is-unique-51hp but it refers to the outdated/legacy/(deprecated ?) version of the documentation.

It's not clear to me how to achieve the same on using the new resolver logic (This: https://docs.amplify.aws/cli/graphql/custom-business-logic/#lambda-function-resolver ).

Can anyone provide a step-by-step example?

Compare the linked legacy guide with the current Amplify custom business logic documentation at docs.amplify.aws/cli/graphql/custom-business-logic/#lambda-function-resolver. Add a step-by-step example for defining a unique model field with Amplify CLI 12.3.0, and verify that the documented workflow matches the current resolver logic.

Written by the indexing model from the issue text.