ENH: Add data type to sparse

[genematx]

Unlike the (dense) ArrayAdapter, adapters for sparse arrays (e.g. COOAdapter) didn't have dtype explicitly stored. This PR adds a new property, data_type, to COOAdapter and the corresponding COOStructure; the new property is initialized similarly to how it's done for ArrayAdapter, including the possibility to use StructDtypes.

The changes are caused by the requirement to specify dtype when exposing the arrays at /zarr/v2 endpoints (see Issue #562 and PR #774).

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section

[danielballan]

I had to think about how this ever worked in the first place! Since we move the data in Arrow IPC format by default, the data types come free. Clearly you are correct: we should be writing down the data type.

Let's merge after some third person has chance to kick the tires and confirm interactively that there are no surprises here.