chore: release 0.6.4

[haverchuck]

Description of changes:
Release 0.6.4

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[ragingsquirrel3]

I agree w Dillon's comment, otherwise looks good. Thanks!

[codecov-commenter]

Codecov Report

Merging #1919 (f28401b) into stable (19870f8) will decrease coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           stable    #1919      +/-   ##
==========================================
- Coverage   48.08%   47.81%   -0.28%     
==========================================
  Files         376      371       -5     
  Lines       10888    10783     -105     
==========================================
- Hits         5236     5156      -80     
+ Misses       5652     5627      -25     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	38.41% <100.00%> (ø)
ios-unit-tests	89.59% <ø> (+0.67%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../amplify/amplify_flutter/lib/src/amplify_impl.dart	65.00% <100.00%> (ø)
...ios/example/ios/unit_tests/AtomicResultTests.swift
..._pinpoint_ios/example/ios/Runner/AppDelegate.swift
...e/ios/unit_tests/MockAnalyticsCategoryPlugin.swift
...y_flutter_ios/example/ios/Runner/AppDelegate.swift
.../ios/unit_tests/amplify_flutter_exampleTests.swift