fix: include only required userAttributes and generate identityPoolName in backend file

[Sanayshah2]

Description of changes

• Filter userAttributes to include only required attributes and not all Standard Attributes in auth/resource.ts
• Set userNameAtttributes to undefined instead of [] in backend.ts
• Modify codegen to include identityPoolName in backend.ts

Description of how you validated changes

Manually creating an application and running migrate command and checking the generated code. Unit tests.

Checklist

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

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