Reader for Hugging Face's SafeTensor format

[TomNicholas]

We should be able to write a new VirtualiZarr reader for the Hugging Face SafeTensors format.

Hugging Face safetensors is an interesting example - it's uncompressed so basically just like reading netCDF3, having no internal chunking. But it also puts all the metadata at the start of the file, making it a bit like cloud-optimized HDF5. See also huggingface/safetensors#527 (comment)

Originally posted by @TomNicholas in #218

cc the Pangeo ML people: @weiji14 @negin513 @maxrjones

[TomNicholas]

The format specification seems very straightforward.

If it really is that simple then I think this reader could potentially be implemented without even using the safetensors library, instead just using fsspec and parsing the bytes ourselves. Our unit tests should still probably check correctness against the safetensors library itself though.

[TomNicholas]

There is an interesting issue on safetensors about "multi-part uploads". Apparently it's not officially supported but nevertheless widespread. This suggests desire for the model weights to be chunked and/or version-controlled, which the virtual zarr approach could obviously help with.