Implement RealtimeAgent for Real-Time Conversational AI Support in ag2 Framework

[sternakt]

This pull request introduces the RealtimeAgent to the ag2 framework, enabling real-time conversational capabilities powered by OpenAI's Realtime API. Key features include:

• Real-Time Audio Interaction: Support for voice-based, real-time conversations integrated with Twilio.
• Agent-Driven Task Management: Seamless task delegation to AutoGen's Swarm teams using the tool-calling mechanism.

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/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.

[sonichi]

Thanks. It'll be helpful to write a short description of the notebook's function and scenario in the beginning.