Currently we have a separate `InstancesDataSource`, which does almost
exactly the same thing as `GeometryDataSource`, except that it hardcodes
a few more columns: "Name", "Rotation", and "Scale". We can easily
replace that with a couple of if statements in the geometry data source.
This also makes named attributes on instances display
in the spreadsheet.
Differential Revision: https://developer.blender.org/D13391
f7f558e293