v3 Arrays should support `get_block_selection`

[rabernat]

The array.get_block_selection capability was added in #991. (See https://zarr.readthedocs.io/en/stable/tutorial.html#block-indexing) It's likely that users may be relying on this. It's useful.

We should consider reimplementing this in the V3 API.

[normanrz]

Fixed by #1917