feat: Add column projection for reading to dask_awkward

[douglasdavis]

This PR shows how to add column projection to uproot.dask when library="ak". For this to work properly right now it would require the changes that are in ContinuumIO/dask-awkward#80 (still WIP). I've made this draft PR just to share the planned interface for making I/O column-project-able. Since uproot.dask(..., library="ak") already uses dask_awkward.from_map(fn: Callable, ...), all fn needs is a project_columns method.