XDM Support for Chatbot Experience Event Schemas

The current XDM schemas are not sufficiently expressive to represent the events emitted during chat interactions between brands and customers. We propose the creation of a new set of generic _chat experience event schemas_ to represent such interactions.

## What are the schemas that are affected by the issue
There are no existing schemas affected - the issue here is that there are no schemas that exist to represent a stream of events that are emitted from a chatbot and/or livechat experience.

## What are examples of products that are impacted by the issue
Our chat automation platform, [Stackchat](https://stackchat.com/), needs to send events to AEP as part of a client project. These types of events are [defined here](https://docs.stackchat.com/Integrations/Cloud-Configurations/Adobe-Analytics/Report-Suite-Configuration.html). On the recommendation of Adobe employee Wouter Van Geluwe (@woutervangeluwe ), we propose creating a set of generic _chat experience event schemas_ that are common to all chat experiences across all third-party chat platforms.

Any chat platform will need to send to AEP both experience events & profile attributes. We plan to fork the XDM repo and raise a PR to add the new _chat experience event schemas_. We will use the existing profile schema for any user profile data.

As a business, we've invested considerable time and resources in defining how chat data should be represented and shared between external systems, as can be seen in our existing third-party integrations, such as with [Adobe Analytics](https://docs.stackchat.com/Integrations/Cloud-Configurations/Adobe-Analytics/Initial-Configuration.html) and Segment. As a thought leader in the space, we're well placed to lead this effort and are looking to the XDM team to provide their blessing for us to proceed.

Review the linked Stackchat event definitions and the existing XDM profile schema to understand the proposed chat experience event model. Confirm the scope and schema design with the XDM team; done would be an agreed set of generic schemas represented in a pull request.

Written by the indexing model from the issue text.