test(core): catch type issues when configuring from external json

[ashwinkumar6]

Description of changes

• Bug fix
  • Update AmplifyOutputs authentication_flow_type type from string literal union to string
• Test
  • Ensure amplify_outputs.json has all combinations defined, else throw type error (CI failure)
  • Ensure TS inferred shape from external json does not throw type issues with Amplify.configure (CI failure)

Description of how you validated changes

linked ^

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

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