fix(amplify-codegen): fix for non-model decoding in flutter v1

[Equartey]

Description of changes

Added the dropped Flutter V1 Model.fromJson() decoding logic back to non-model embeds. Customer observed embedded non-model types not populating values after requests were made. This adds a fallback to previous logic.

Issue #, if available

aws-amplify/amplify-flutter#4872

Description of how you validated changes

Validated decoding against this schema in both Flutter V1 and V2.

type MyModel @model @auth(rules: [{ allow: owner }]) {
  id: ID!
  owner: String
  token: Token
  tokens: [Token]
}

type Token {
  externalId: String!
  value: String!
}

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

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