ENH: adding Gator derived dist column to TAP queries via a keyword

Gator provided some derived columns that are not returned in the default TAP response when querying. On the astroquery level the backend is transparent, so we may actually add an enhancement where we return at least some of the derived columns as part of an extended query. I would say we should only do this as an opt-in and only in the higher level API, e.g. `query_region` and not as part of `query_tap`.

Start by locating the higher-level query_region API and comparing it with query_tap. Determine which Gator-derived columns and opt-in keyword should be supported; done means the selected derived columns appear through the higher-level API without changing query_tap.

Written by the indexing model from the issue text.