Variable-Sized Data Types handling

[ilan-gold]

Once this lands in zarr-python we should update here, if I understand things correctly.

zarr-developers/zeps#47

[LDeakin]

This shouldn't be too hard on the zarrs-python side:

• Convert Python variable length arrays to the zarrs representation ArrayBytes::Variable
• Search for into_fixed(), new_flen() and add variable length support

There is an incompatibility between zarrs and zarr-python for variable-sized binary data that I am inclined to address in the next release. zarrs uses the Binary data type (matching ZEP0007) and zarr-python uses Bytes.