r/sdplyr: Can `collect()` return an `sf` object by default?

I know this would be probably more complex than it sounds.

- The result data frame might not contain a geometry column. If we naively attempt to create an sf object, it fails when there's no geometry column. We need to fall back to tibble.
- The result data frame might contain multiple geometry columns. We might need to provide some option to specify which column is primary.
- Considering SedonaDB has both vector and raster, honestly I'm not sure sf is the appropriate representation.

Start by tracing the R sdplyr collect() entry point and reviewing how its result data frame exposes geometry columns. Resolve whether sf is appropriate for vector and raster results, including missing or multiple geometry columns, before defining the default and fallback behavior. Done means the representation and primary-geometry behavior are specified and covered for these cases.

Written by the indexing model from the issue text.