Addition of Summary Metrics class

## What are the schemas that are affected by the issue
New Summary Metrics schema - https://ns.adobe.com/xdm/context/summarymetrics - extending the existing time-series behavior along with a new descriptor - https://ns.adobe.com/xdm/descriptors/descriptorTimeSeriesGranularity - that provides the specifics for the granularity of the summary.

## What are examples of products that are impacted by the issue
This class will be used for Adobe Customer Journey Analytics, Adobe Intelligent Services, Adobe Experience Platform ingestion of summary metrics, and additional future Adobe products dealing with summary data vs raw events.

Start by reviewing the existing time-series behavior and the linked Summary Metrics and descriptorTimeSeriesGranularity schema definitions. Done means the new class and granularity descriptor are represented consistently for the listed Adobe products; the issue names no files or tests, so confirm scope with maintainers first.

Written by the indexing model from the issue text.