insights: new streaming search decoder to support aggregation

Created by: chwarwick

Create a new streaming search decoder to support aggregating search results.

In initial implementation it should be possible to aggregate by Repo, Path, and Author. This decoder should be extensible to add additional aggregation types in the future, for example aggregations by capture group matches.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin