feature: stringMax, stringMin aggregations

[ColeAtCharter]

Description

The stringMin/stringMax aggregation should return a string which can be filtered on without finalization, unlike a complex aggregation like stringFirst/StringLast. It should be usable at ingest and at query time. Eventually, the aggregator could include a configuration property for the collation to be used for the comparison.

This implies the ability to have string metric columns which could eventually allow users to create custom aggregations which can be directly consumed without finalization, without special serializing/deserializing, and without writing custom logic (eg, extensions)