feat: Auth codegen

[Sanayshah2]

Description of changes

Added OAuth flows, Oauth scopes, Unauthenticated logins, Custom Attributes, Group precedence, read/write permissions for attributes, User pool name, oidc/saml external providers codegen

Some features are natively supported; for those not supported natively, Property Overrides are used to mimic the same behavior.

Description of how you validated changes

Unite tests for various scenarios and edge cases. Also, manually verified against a deployed application and checking for generated code having relevant property overrides.

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] Tests are changed or added
• [x] Relevant documentation is changed or added (and PR referenced)
• [x] New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• [x] Pull request labels are added

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