[AI] Add FirebaseAI.generativeModelSession(...)#15913

andrewheard merged 5 commits intoah/ai-generative-model-sessionfirebase/firebase-ios-sdk:ah/ai-generative-model-sessionfrom

ah/ai-generative-model-session-apifirebase/firebase-ios-sdk:ah/ai-generative-model-session-apiCopy head branch name to clipboard