firebase_vertexai docs do not illustrate how to play nice with App Check

[wasserman]

The docs don't include how to use firebase_appcheck with firebase_vertexai even though they recommend to do so.

At https://pub.dev/packages/firebase_vertexai/example it shows the following:

var vertex_instance = FirebaseVertexAI.instanceFor(auth: FirebaseAuth.instance);
final model = vertex_instance.generativeModel(model: 'gemini-1.5-flash');

At https://firebase.google.com/docs/vertex-ai/get-started?platform=flutter#initialize-service-and-model it shows this:

final model = FirebaseVertexAI.instance.generativeModel(model: 'gemini-2.0-flash');

It would be helpful to include the case where you have App Check enabled:

model = FirebaseVertexAI.instanceFor(appCheck: FirebaseAppCheck.instance).generativeModel(model: 'gemini-2.0-flash');