Demo: add support for reasoning models.

[nezhyborets]

Is your feature request related to a problem? Please describe.
Currently Demo is built on streamable version of chat completions. Thus, it's impossible to select reasoning models like o1 or o1-mini to try them out. We need to that ability.

Describe the solution you'd like
I'm not sure if view that works with stream: false should be completely separate from existing one. That is the matter for discussion.