Implement Variable Chunking (ZEP003) in V3

[jhamman]

https://github.com/zarr-developers/zarr-python/pull/1483 provides a proof of concept implementation of Variable Chunking 🙌 ! That implementation is on top of V2 so will obviously need to be ported to V3. This issue flags that some design considerations in the codec pipeline are going to be required and that input on #1583 (and initial PRs like #1588 and #1589) would be useful. To close this issue, we should have a working implementation of the ZEP003 in the v3 dev branch.

cc @martindurant, @ivirshup, @agoodm, @alex-s-gardner, @d-v-b, @normanrz

[martindurant]

That implementation is on top of V2 so will obviously need to be ported to V3

It actually works with V3 as given, for the versions of V3 coexisting with V2 in this repo.