Added info for Group and Array

[TomAugspurger]

Quick pass at adding .info to Group and Array. Like 2.x, info is a property, but it will only contain information that's known statically. I've added a Group.info_complete method that actually calls the store to get dynamic information (for group this is children).

I've punted on a similar Array.info_complete. For that, we'll need to expand the Store ABC to give us an nbytes method to get the size of the array in bytes. Maybe similar for the actual number of chunks written. Doable, but separate from this PR I think .

Closes https://github.com/zarr-developers/zarr-python/issues/2135