feat(authenticator): Handle SignedIn events from outside of Authenticator

[mattcreaser]

• PR conforms to Pull Request guidelines.

Issue #, if available: #235

Description of changes:
Authenticator now reacts to SIGNED_IN events it receives from Amplify. Historically it didn't do this because the idea was that you would only use Authenticator for Sign In, and not use Amplify APIs directly, but #235 rightly points out that you currently would use Amplify for social sign in via Cognito's web UI, which would leave Authenticator in an incorrect state.

In this PR Authenticator listens to events from Amplify for when the user signs in. If the event is not expected (i.e. Authenticator is not currently in the process of calling a sign in API) then it reacts to the sign in by going directly to the SignedInState.

How did you test these changes?

• Unit tests
• Test app with programmatic sign in (Amplify.Auth.signIn)
• Test app with WebUi sign in (Amplify.Auth.signInWithWebUI)
• Test app with social sign in (Amplify.Auth.signInWithSocialWebUI)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(liveness): message, fix(authenticator): message, fix(all): message)

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

[mattcreaser]

This is a Draft PR pending verification of the web UI and social web UI use cases. All test cases have been validated.

[codecov]

Codecov Report

Attention: Patch coverage is 53.33333% with 14 lines in your changes missing coverage. Please review.

Project coverage is 26.11%. Comparing base (8682849) to head (b5f1861).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
+ Coverage   25.90%   26.11%   +0.21%     
==========================================
  Files          95       95              
  Lines        4458     4468      +10     
  Branches      865      869       +4     
==========================================
+ Hits         1155     1167      +12     
+ Misses       3113     3109       -4     
- Partials      190      192       +2     

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