feat: enable manual column optimization with dask-awkward

[pfackeldey]

Enables dask-contrib/dask-awkward#565 for uproot.dask.

Example:

import uproot
import dask_awkward as dak


# foo.root has 3 branches: "A", "B", "C"
array = uproot.dask({"foo.root": "tree_name"})

# get dask key in graph:
key, _ = next(iter(array.dask.keys()))

# manual column optimization for "A" and "C"
needs = {key: ["A", "C"]}
optimized_array = dak.manual.optimize_columns(array, needs)

# this only loads "A" & "C" from disk in a single read call (`tree.arrays([...])`), 
# it doesn't load "B", and avoids typetracing entirely (it's already optimized...)
concrete_array = optimized_array.compute()

key corresponds to the key of the from-uproot IO layer. needs can have multiple keys that point to various projectable IO-layers.

[ianna]

@pfackeldey - Thanks! Looks good to me. Please, merge it. Thanks.