Support Zarr format v3 as part of the API

[jeromekelleher]

Currently we're using an environment variable to globally switch between zarr format v3 and v2. We will need to support both indefinitely, so we need to bake things in now to the API.

I suggest we add a common --zarr-format=2 option to the CLI, and a zarr_format=None parameter to API functions that need it.

I don't think we need to exercise the whole test suite on both versions, we just pick a few examples that'll exercise the main code paths.

[tomwhite]

I don't think we need to exercise the whole test suite on both versions, we just pick a few examples that'll exercise the main code paths.

I'd like to keep the ability to run the whole suite against both versions, at least for a bit longer, as I worry there are subtle corner cases that will sneak in and we won't detect immediately.

[jeromekelleher]

Addressed in #483, but keeping this open until we have documented in the CHANGELOG