base64 encode fill value for some dtypes with zarr_format=2

[TomAugspurger]

Closes #2282

Needs more tests.

• 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)

[TomAugspurger]

This should be ready to go (the CI failure is from appveyor).

[d-v-b]

were we not doing this in the v2 codebase? I couldn't find code for it.

[TomAugspurger]

I found it at

zarr-python/zarr/meta.py

Line 215 in 6b9ab9e

def decode_fill_value(cls, v: Any, dtype: np.dtype, object_codec: Any = None) -> Any:

(and borrowed the implementation).

[jakirkham]

Thanks Tom! 🙏

JFYI we have a Base64 codec in Numcodecs, perhaps it is worth using to align on behavior

[TomAugspurger]

Thanks John. I think this version (rather than numcodec's) is OK. It ends up using the same actual implementation of base64.standard_b64encode but it's slightly more direct with the calls directly in the serialization code in this PR.