insights: extend aggregations decoder logic for group by "date"

Created by: chwarwick

The "Group By" insight supports grouping by date because of the plan to convert that insight to using the aggregation search logic, a new SearchAggregationMode needs to be added to support grouping by "date".

See existing grouping logic here

/cc @joelkw @felixfbecker @vovakulikov @unclejustin