Update OpenAI Assistante Agent on_reset

[lspinheiro]

Why are these changes needed?

This PR introduces the following changes for the OpenAI Assistants API agent

1. Update the on_reset method to present the initial thread state and only delete new messages
2. Move optional dependency imports to conditional blocks to avoid module import errors
3. Add aiofiles to the openai extra
4. Remove deprecated extras (missed in the clean up)

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [ ] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.