V3 Consolidated Metadata

[joshmoore]

follow on to: #1142
background reading: zarr-developers/zarr-specs#136

I would like to propose that we remove the machinery for consolidated metadata in the current V3 implementation in favor of an upcoming specification. This would include:

• dropping zarr._storage.v3.ConsolidatedMetadataStoreV3
• disallow version=v3 in zarr.convenience.consolidated_metadata

[jhamman]

Noting that what we really need is someone to work on:

• A spec for consolidated metadata in v2
• A spec for consolidated metadata in v3
• An implementation in the active v3 branch

Marking this with a help-wanted label.

[jhamman]

Noting that @TomAugspurger has been making solid progress here. See also zarr-developers/zarr-specs#309