New API to recompute a column in a dataset 

The new route is /compute/{resource}/{column}, where {resource} is the dataset resource name and {column} is the computed column that should be recomputed. The API does not currently support recomputing only specific row IDs - only all rows in the dataset.

Source columns + PK column are exported from truth (I had to add support for requesting specific columns in the export), then we follow the same computation code path as regular upserts. Once the column values have been recomputed, the augmented rows are upserted.