ENH: add search offset column to Heasarc.query_region

The returned table does not have a an offset or distance from the requested RA and DEC.
The end user can work it out using `SkyCoord`, but it would nice to have it as an extra column

Start at the Heasarc.query_region entry point and inspect how its returned table is assembled. Determine the intended offset or distance representation from the requested RA and DEC, then verify that the result includes this value as an extra column.

Written by the indexing model from the issue text.