native support for custom attributes on auth

[ykethan]

Environment information

System:
  OS: macOS 14.4.1
  CPU: (8) arm64 Apple M1
  Memory: 210.19 MB / 16.00 GB
  Shell: /opt/homebrew/bin/fish
Binaries:
  Node: 20.11.1 - ~/.local/share/nvm/v20.11.1/bin/node
  Yarn: 1.22.19 - ~/.local/share/nvm/v20.11.1/bin/yarn
  npm: 10.2.4 - ~/.local/share/nvm/v20.11.1/bin/npm
  pnpm: 8.15.4 - ~/Library/pnpm/pnpm
NPM Packages:
  @aws-amplify/backend: 1.0.2
  @aws-amplify/backend-cli: 1.0.3
  aws-amplify: 6.3.2
  aws-cdk: 2.141.0
  aws-cdk-lib: 2.141.0
  typescript: 5.4.5
AWS environment variables:
  AWS_STS_REGIONAL_ENDPOINTS = regional
  AWS_NODEJS_CONNECTION_REUSE_ENABLED = 1
  AWS_SDK_LOAD_CONFIG = 1
No CDK environment variables

Description

Followup on issue: https://github.com/aws-amplify/amplify-backend/issues/1511

we currently need to modify the cfnUserPool schema to add a custom property on the backend. would be great if we could add this on defineAuth directly.