Support stream responses for chats and models generate content APIs

[RulaKhaled]

1. https://googleapis.github.io/js-genai/release_docs/classes/models.Models.html#generatecontentstream
2. https://googleapis.github.io/js-genai/release_docs/classes/chats.Chat.html#sendmessagestream

[linear]

JS-940 Support stream responses for chats and models generate content APIs