widen fill value of bytes dtype

[d-v-b]

This PR changes the bytes dtype to support a base64-encoded string as a fill value. This change is motivated by compatibility with the fill values encoding used for variable-length bytes in Zarr V2 data, and also the current behavior of the variable_length_bytes data type in zarr-python, which I would like to deprecate in favor of bytes.

I also re-structured some of the spec for clarity.

xref zarr-developers/zarr-python#3559

I am opening this as a draft because there is content missing from this PR -- given that we have 2 different fill value encodings, we should provide a recommendation for a default when writing data. I'm happy choosing either form as the default.

[LDeakin]

+1 for this

should provide a recommendation for a default when writing data

Base64 is more compact and I'd say should be preferred. But in the near term, implementations probably want to stick with the array form until there is more implementation support.

[d-v-b]

Base64 is more compact and I'd say should be preferred. But in the near term, implementations probably want to stick with the array form until there is more implementation support.

I added a soft recommendation for the base64 version

[d-v-b]

I think this is ready for review