insights: add group by field for compute type insight to current line chart API

Created by: coury-clark

For the first version of the compute feature in Code Insights we are going to retrofit the bar chart / compute mapping type into the search insight line chart API.

One assumption here is that we are going to take in a query string without the mapping syntax and modify it to map by value. For example: mystring lang:go group by language becomes content:output(mystring -> $lang) lang:go

This issue isn't concerned with that mapping, but modifying the API to support this mapping.

1. Introduce a field in the input to group by lang, author, path, repo, date, etc.
2. Modify the response to include the grouped field