DecisionEvents must be countable, group-able and aggregate-able by channel

## What are the schemas that are affected by the issue
https://ns.adobe.com/experience/decisioning/decisionevent ->
https://ns.adobe.com/experience/decisioning/proposition-details ->
https://ns.adobe.com/experience/decisioning/proposition-detail

## What are examples of products that are impacted by the issue
Platform Decisioning, Campaign

DecisionEvents must be countable, group-able and aggregate-able by channel.
The channel is related to the activity and option selections and therefore the logical place for recording the channel context for the decision events is proposition detail.
Note that there can be multiple propositions made with a single decision and those propositions don't have to be for the same channel. E.g. fatigue management decisions will span multiple channels.

Start by reviewing the affected decisionevent, proposition-details, and proposition-detail schemas linked in the issue, along with the examples of impacted products. Define how channel context should be represented when one decision contains propositions for multiple channels, then confirm the schemas support counting, grouping, and aggregation by channel.

Written by the indexing model from the issue text.