[MLv2] Full `:column`s for `:dimensions` in `available-drill-thrus` (#34810)

Previously only the `:column-name` was provided, and there is a
self-proclaimed "icky hack" to guess at the full columns. The FE
provides the (JSON) columns, so this PR converts and uses them.

The icky hack can be removed.

`:column-ref` is included because sometimes it's important that the refs
use the same UUID.