Add integration tests in flutter/flutter for extension safe framework:

From design doc: https://flutter.dev/go/app-extensions
In the framework repo, a non-Flutter iOS app implementing a share sheet will be added to the framework integration test directory to serve as the reliable “host app”. This will exercise a new Flutter integration test app with the Share sheet implemented. A test script will install the Flutter app (and thus the share plugin), and a XCUITest will launch the host app, navigate to the share sheet, and select the Flutter app share option. Basic functionality in the Flutter share extension will be smoke tested.

part of #124287