Make things that return coordinates return SkyCoord objects somehow

I'm not sure exactly what the scope of this issue might be, but I think it's worth asking at least. It appears to me that if I do something like querying `Vizier` for catalogs of objects, while I can return their standardized _RAJ2000/DEJ200 columns, there's no straightforward way to get back a `SkyCoord` for the catalog without manually creating one myself. Has there been any discussion about whether this is desirable? I can think of some clear ways to do it in e.g. `Vizier` or `SDSS`, at least now that astropy tables support returning tables with columns that are objects... But I'm not sure how general this should be. Is there someplace that could be plugged into that would allow _all_ astroquery tables with appropriate column names or something to get an added skycoord column? For that matter, is this desirable?

cc @keflavich

Start by reviewing how Vizier and SDSS return catalog tables, especially the _RAJ2000 and _DEJ200 columns, and how astropy tables support object-valued columns. Compare whether a shared astroquery entry point could add SkyCoord columns across services. Done requires agreeing the general scope and supported column conventions, then verifying the chosen behavior on representative catalog results.

Written by the indexing model from the issue text.