Add onCallGenkit extension

[mosuem]

Add https://firebase.google.com/docs/reference/functions/2nd-gen/node/firebase-functions.https#httpsoncallgenkit

Re-opening of #26

[gemini-code-assist]

Code Review

This pull request introduces the firebase_functions_genkit package, which provides an extension to integrate Genkit flows with Firebase Functions, supporting both streaming and non-streaming callable requests. The review feedback identifies critical issues regarding type safety and asynchronous execution, specifically the need for a generic Input type in onCallGenkit, missing await statements on Future results, and syntax errors in the example's schema definitions.