Support openai assistant api

[gagb]

Why is this needed?

OpenAI Assistants are out and we should integrate them in AutoGen ASAP!

### Tasks
- [ ] https://github.com/microsoft/autogen/pull/616
- [ ] https://github.com/microsoft/autogen/pull/632
- [ ] https://github.com/microsoft/autogen/pull/636
- [ ] https://github.com/microsoft/autogen/issues/642
- [ ] https://github.com/microsoft/autogen/issues/643
- [ ] https://github.com/microsoft/autogen/issues/637
- [x] Add groupchat notebook for gptassistant
- [x] add links to all the notebooks in the website and add link to blogpost in readme
- [x] add the test to contrib openai test
- [ ] https://github.com/microsoft/autogen/issues/641
- [ ] https://github.com/microsoft/autogen/issues/686
- [ ] https://github.com/microsoft/autogen/pull/665
- [ ] https://github.com/microsoft/autogen/pull/718
- [ ] https://github.com/microsoft/autogen/issues/916

[DementedWeasel1971]

Also allow for the use of threads as part of enabling mutli-agent engagement.

[gagb]

Relevant PR that will be reviewed and improved: #616

[gagb]

@LeoLjl @qingyun-wu I updated the roadmap with pending tasks.

[gagb]

Also allow for the use of threads as part of enabling mutli-agent engagement.

This was added in #616 and #632. Please let us know if you see issues!