Expose widgets for automation testing

[amin-nas]

Enable app developers to write automate tests by accessing Ensemble widgets.

example test

  final Finder textLoginFailedFind = find.byKey(
      ValueKey(IntegrationTestKeys.loginScreenKeyUtils.key_error_popup_text));

[TheNoumanDev]

Implementation Strategy

• Leverage existing testId property in Ensemble widgets or add new property name Key
• Wrap returned widget with KeyedSubtree, using testId as its key as we doing it for visible property.

Tentative working example

LoginScreen:
  body:
    TextInput:
      testId: username_input

final usernameInput = find.byKey(ValueKey('username_input'));

Estimated time: Would take easily 1-2 days with proper testing with above appraoch.
CC: @amin-nas