Adopt streaming in Demo app

[DJBen]

What

Adopt streaming in demo app, and add a model switcher between gpt3 and 4 models.

DemoAppStreaming.mov

Why

Demo app will be broken if this doesn't come in. Also I think this is how people want to use the API primarily.

Affected Areas

Demo app.

[rotemdoron]

any plan to include this?

[DJBen]

@rotemdoron I do. Ultimately I defer to @Krivoblotsky

[Krivoblotsky]

I’m ok to merge it into the streaming branch, however, I’m gonna run more tests before merging streaming branch itself to handle some corner cases.