Add zarr.abc.store.Store.get_many, a bulk counterpart to Store.get that
retrieves many values — each a whole key or a (key, byte_range) pair — in a
single call. It generalizes Store.get_ranges (many ranges of one key) to many
keys, yielding (request_index, Buffer | None) batches in completion order so a
store can coalesce reads that land in the same underlying object. The method is
defined on the Store ABC with a default implementation that fetches the
requests concurrently with Store.get, so every store inherits a working
version; stores whose backend can retrieve many objects together should override
it (FsspecStore does, coalescing via fsspec's cat_ranges). Coalescing
tuning is left to each store rather than exposed on the interface. This restores
and generalizes the batched-fetch capability of the v2 getitems Store API.