add bytes dtype

[d-v-b]

This PR adds a Bytes dtype that is nearly identical to the existing VariableLengthBytes dtype save a few differences:

• the V3 JSON form is "bytes" instead of "variable_length_bytes"
• The fill value representation is an array of ints, instead of a base64-encoded string
• Bytes is consistent with a published spec, instead of not being described by a spec.

The latter point is the most important.

Because Bytes is nearly identical to VariableLengthBytes, it could be a drop-in replacement for VariableLengthBytes, save for the JSON fill value encoding differences between the two codecs, which raises two questions:

• is base64 encoding a bytestring a better or worse JSON serialization than representing the same bytestring as an array of integers?
• could (or should) we amend the bytes data type spec to recommend reading (or reading and writing) both fill value encodings? If so, the bytes data type and the variable_length_bytes data types could be fused completely.

Since zarr python 2.x was saving bytes fill values as base64-encoded strings, there's a bit of inertia there. Would also be good to hear thoughts from other implementers (@LDeakin , @jbms, @manzt )

[codecov]

Codecov Report

❌ Patch coverage is 46.47887% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.66%. Comparing base (b3e9aed) to head (9ff7ec4).

Files with missing lines	Patch %	Lines
src/zarr/core/dtype/npy/bytes.py	46.37%	37 Missing ⚠️
src/zarr/core/dtype/npy/common.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3559      +/-   ##
==========================================
- Coverage   61.77%   61.66%   -0.11%     
==========================================
  Files          85       85              
  Lines       10165    10233      +68     
==========================================
+ Hits         6279     6310      +31     
- Misses       3886     3923      +37     

Files with missing lines	Coverage Δ
src/zarr/core/dtype/npy/common.py	61.53% <50.00%> (-0.19%)	⬇️
src/zarr/core/dtype/npy/bytes.py	51.12% <46.37%> (-1.88%)	⬇️

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