Implement ZstdZarrCompressor

[mkitti]

This implements ZstdZarrCompressor which wraps around CodecZstd as a package extension.

Part of the complication of using package extensions is getting a reference to new types defined in the extension. I created a mechanism by which you could specify the compressor as a string, which would then lookup the type from a dictionary.

I'm also wondering if there might be a general way to wrap TranscodingStreams codecs into Zarr compressors.