Allow user sign in by Cognito hosted ui without credentials

[powerful23]

Issue #, if available:
#705
Description of changes:

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

[codecov-io]

Codecov Report

Merging #710 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #710      +/-   ##
==========================================
- Coverage   87.06%   87.03%   -0.03%     
==========================================
  Files          74       74              
  Lines        3494     3495       +1     
  Branches      668      668              
==========================================
  Hits         3042     3042              
- Misses        435      436       +1     
  Partials       17       17

Impacted Files	Coverage Δ
packages/aws-amplify/src/Auth/Auth.ts	88.52% <0%> (-0.15%)	⬇️

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[richardzcode]

👍

[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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