replacing `fromJSON` with `Rarr::read_zarr_attributes`

[Artur-man]

It would be ideal to use Rarr::read_zarr_attributes instead of fromJSON which is used throughout the package.

However, the default arguments of fromJSON include default simplifications of the list, which creates small data.frames in the list:

> fromJSON
function (txt, simplifyVector = TRUE, simplifyDataFrame = simplifyVector, 
    simplifyMatrix = simplifyVector, flatten = FALSE, ...) 

Rarr::read_zarr_attributes however does not do this, thus tests need to be updated if we were to migrate to Rarr based attributes.