Time-series contains event type

## What are the schemas that are affected by the issue
https://ns.adobe.com/xdm/data/time-series

The time-series schema enforces an event type when it is not really necessary. Others may not be able to create a timeseries structure with their own set of event types. Ideally, the time series structure should just have a timestamp field and is an abstract class. If we named this as a base event schema where there is a generic event id, event timestamp and event type, that would likely work better for the future consumption of this schema.

Start by reviewing the time-series schema at https://ns.adobe.com/xdm/data/time-series and its current event-type requirement. Done should be a resolved schema design that permits time-series structures without a mandatory event type and clarifies whether a generic base event schema is needed.

Written by the indexing model from the issue text.