Use implicit fill values for zarr v2

[TomAugspurger]

Closes #2271

I'm not sure if this is a good idea, but this, consolidated metadata, and #2270 lets xarray round trip a dataset. The basic idea is to let ArrayV2Metadata represent the v2 metadata a bit more faithfully. Currently, we "eagerly" interpret the lack of fill_value in a V2 document as the default fill value for some dtype (e.g. 0 for int64). This PR changes that, to allow ArrayV2Metadata.fill_value to be None if it's unspecified in the metadata. But we do need some fill value when reading data. So that's where we finally substitute in the default.

This doesn't really affect Zarr directly, but it is useful for downstream libraries that want to inspect the array metadata before reading and make some decision based on whether or not fill_value is specified.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[jhamman]

A few comments. Seems reasonable though.

[rabernat]

This looks good. My one suggestion would be to consider a special case for object / string dtypes.

[rabernat]

Awesome, nice work Tom!

[jhamman]

Something we may consider in the future is to parameterize the CodecPipeline class with the zarr_format of the calling Array. This would allow us feel more confident that workarounds like this one only apply to v2 data.

[jhamman]

Thanks @TomAugspurger!