feat(angular): add support for onpush change detection strategy

[esauerbo]

Description of changes

Changes authenticator component to use OnPush strategy, which means it can be consumed by components that use either OnPush or Default strategy.

e.g.

@Component({
  selector: 'sign-in-with-email',
  template: '<amplify-authenticator></amplify-authenticator>'
})

works the same as

@Component({
  selector: 'sign-in-with-email',
  template: '<amplify-authenticator></amplify-authenticator>'
  changeDetection: ChangeDetectionStrategy.OnPush
})

Because authenticator now uses OnPush, we run change detection manually (changeDetector.detectChanges()) whenever the authService updates. This is achieved by creating an observable from the auth service

this._authStateObservable$ = from(this._authService);

using rxjs, which can then be natively subscribed to emit events when authService updates.

Issue #, if available

fixes #4899

Description of how you validated changes

Checklist

• Have read the Pull Request Guidelines
• PR description included
• Relevant documentation is changed or added (and PR referenced)
• yarn test passes and tests are updated/added
• No side effects or sideEffects field updated

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

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[esauerbo]

We no longer return the facade from the subscribe function, instead it just invokes a callback so testing that the callback gets called.

[esauerbo]

BehaviorSubject is a type of observable that takes an initial value and emits current value to observers. Using it with the async pipe like"qrCodeSource$ | async" will automatically subscribe to new values and mark component for check when qrCodeSource changes, and will automatically unsubscribe when component is destroyed. It's a cleaner way of subscribing to an observable and then running change detection.