Introduce a raw count aggregation type to the field aggregations

Sometimes you want to display the count of rows in a table or something similar. It would be great to use the aggregation values from the views to display summary data.

Ref: https://community.baserow.io/t/is-there-a-way-to-display-number-of-rows-records/5675

We simply want to introduce a COUNT(columnName) aggregation so that users can return a raw count of rows, regardless of whether values are set or not.