Add support for more Assistants API features

[bwhtmn]

What

• Add support for Assistant API features:
  • function calls
  • create & run thread in a single request
  • submit tool outputs (i.e. function call responses)
• Updated the demo app and unit tests to cover the newly added functionality noted above.
• Updated the chat completions API to use the new "tools" parameter instead of the deprecated "function_call" parameter, for a better match with the way that the Assistants API calls functions.

Why

I was working on an OpenAI Assistants API use case, and I noticed that you had already implemented almost all of the Assistants API features except for function calls, which was the main thing that I needed. So I decided to branch off your open PR and add that functionality. Perhaps this could help in getting your PR merged since it seems to have stalled out for awhile?

Affected Areas

Assistants API. Chat Completions API.