Handle fill_value for bytes dtype

[TomAugspurger]

This partly fixes how we handle bytes dtype arrays. Pushing up early for a bit of a discussion around the spec:

AFAICT, bytes aren't a "required"(?) dtype for zarr v3. From https://zarr-specs.readthedocs.io/en/latest/v3/core/v3.0.html#fill-value:

Extensions to the spec that define new data types must also define the JSON fill value representation.

So the proper way to do this would be to

1. Define a zarr extension for bytes dtype (in this case, fixed-width null terminated bytes). Probably need to do the same for unicode
2. In the extension, define how the fill value should be encoded in JSON. Presumably as base64 encoded.

[TomAugspurger]

In general, we can't assume bytes are valid JSON so they must be transformed somehow. base64 encoding seems as good as anything else.

We'll need to update the from_dict to do the decoding. We probably need to see that we're decoding to a bytes dtype and use that when parsing the fill value.

[jhamman]

🤦 - we still don't have a string dtype in the spec! In #2209, I implement a dtype validator that would raise an error if this was passed in.