Store ome-zarr external JSON metadata somewhere other than array's attributes

[andy-sweet]

Currently on main, we allow users to pass through external/app-specific JSON metadata when write to zarr. This gets written to the .zattrs JSON text file associated with the full-resolution zarr array.

While this does not conflict with v0.4 of the OME-NGFF specification (i.e. they do not reserve the array attributes for anything in particular), we may want to move this external metadata to be somewhere else for general hygiene and to avoid conflicts in the future.

For example, we could write something like acquire-metadata.json in the same directory as the top-level group in the ome-zarr directory.