Allow `distinct_on` to be able to handle multiple fields

Right now the `distinct_on` call only allows for a single column to be defined. We had an issue in our project today that caused us to have to go well out of our way to be able to get the data we wanted. If `distinct_on` supported the multiple columns, it would have been a very quick change.

https://www.postgresql.org/docs/current/sql-select.html#SQL-DISTINCT

Start by locating the `distinct_on` entry point and its existing tests, then read the linked PostgreSQL DISTINCT documentation for the multiple-field behavior. Confirm how the current single-column call is represented and define done as supporting multiple fields with coverage for the new usage.

Written by the indexing model from the issue text.