Remove responsibility for managing readonly flag from abstract Store hierarchy?

[TomNicholas]

Copying @paraseba's suggestion from pydata/xarray#10457:

Long term, I'd love to see the responsibility for managing the readonly flag removed from the abstract Store hierarchy. It feels this is a concern that is beyond what a Store should know about. It would help us keeping the Stores abstract and focused. Readonly-ness sounds like a session concern, independent of the details of how to write zarr objects to different media.

cc @maxrjones @d-v-b

[d-v-b]

sounds a lot like a request for permissions to be associated with arrays and groups instead of the store. cc @jhamman