[extension codecs] zarr v2 vlenutf8 codec

[d-v-b]

For zarr v2 data, zarr-python supported variable length strings via the numcodecs VLenUTF8 codec. Support for this codec has been added for zarr v3 data: zarr-developers/zarr-python#2036. This codec is not listed in the zarr v3 spec, but it should have a specification document hosted in this repo, so other zarr v3 implementations can easily use it. This issue tracks the creation of such a specification document.

cc @rabernat

It would also be great to get #312 resolved, so that the context for this codec (and other extension codecs) is codified in the spec.