Implement `Feedback` feature

[nivisi]

The Android SDK opens a separate activity to collect feedback data and send it to NStack. In the Flutter world it is a route. But as we want to make the UI as flexible as it is, building a preconfigured page doesn't seem to be the solution.

Of course, we can create an NStackFeedbackForm widget with various configs (like textFieldBuilder: (BuildContext, ValueChanged<String> onChanged), appBarBuilder:, sendFeedbackButtonBuilder:). But it looks like an overkill.

So perhaps it would make sense to only provide the API to send the feedback, e.g. NStack.feedback.send(...) function.

@johsoe what do you think?