Add Conversational Experiences to XDM

As LLMs and chat interfaces have become more popular over the years, it is becoming increasingly important to be able to model and store these conversations.

We should propose a general purpose conversational experience data model.

No file, test, or entry point is named. Review existing XDM data models and JSON schema conventions first, then define the scope and requirements for a general-purpose conversational experience model. Done means a proposed model is documented clearly enough for project review.

Written by the indexing model from the issue text.