Draft: Add mode to read consolidated ZARR datasets

[mannreis]

This changes add a mode option mode=consolidated (perhaps best to do it by default when reading and fallback if fails) that will fetch a possibly existing .zmetadata file from the root of the dataset. That could serve as unified representation to be used whenever needing group or variable metadata further down the code path.

This is a WIP motivated by #2987 and lacks (at least):

• Unit testing
• Functional testing
• Support Zarr V3
• Robustness when open consolitaded not available

[DennisHeimbigner]

The way I planned to do the consolidated metada (aside: would like a shorter term than "consolidated")
for netcdfd is to create another dispatch layer for accessing various metadata pieces.
So for v2, this would wrap read/write of .zroup .zarray and .zattrs.
For v3, this would wrap access to zarr.json.