En route to replacing ZArray custom class with zarr-python's ArrayV3Metadata, we first need to support ManifestArrays using ArrayV3Metadata.

In order to achieve #411 in an incremental way, my proposal is to first support ArrayV3Metadata as well as ZArray in ManifestArray's, so that we can then incrementally refactor the readers and writers to implement or use ManifestArray's using ArrayV3Metadata while still having passing tests. Once all readers and writers using ManifestArrays are refactored, we can remove the custom ZArray class.

As suggested in #411, this will include renaming the ManifestArray zarray property to metadata.

See also #423 for a reference on how codec pipelines may be handled.

Thoughts welcome, cc @mpiannucci @TomNicholas