Rework code generator to use the Smithy models

[xvrh]

Currently, in the code generator, we use the json models from the Javascript SDK v2: https://github.com/aws/aws-sdk-js/tree/master/apis

Those are deprecated (not updated anymore) and replaced with Smithy
All the models are available as "json ast": https://github.com/aws/api-models-aws

We should update the generator to this new format if we want to continue to update the clients.

[Schwusch]

AWS themselves has done some work: https://github.com/aws-amplify/amplify-flutter/tree/main/packages/smithy/smithy_codegen

[xeard]

Hello, I am using this project in Production and would like to see it maintained with the latest versions of AWS services.
Please consider making the migration to Smithy models.
Much appreciated.