Implement partitioned AwkwardArrays and a dask-awkward client

[danielballan]

Based on a quick reading of dask-awkward, AwkwardArrays are partitioned along the length of the AwkwardArray. This is simpler to describe that I naively expected. Similar to how Tiled describes DataFrame "partitions" (the 1-dimensional analogue of "chunks"), it can describe Awkward partitions.

• Add npartitions to the AwkwardStructure, matching the term we use in TableStructure. Default is 1 for back-compat.
• Add npartitions query parameter. It really ought to be required, but we can default for 1 for back-compat, at least temporarily.
• Add a DaskAwkwardClient.