Recursive explore for listable DataVaults

[normanrz]

Some storage backends, e.g. S3, GCS, local file system, support directory listing. On these storage backends, it would be great if we could run the dataset exploration recursively to capture nested or multiple layers into one dataset.

For example, this dataset is addressed by s3://janelia-cosem-datasets/jrc_mus-nacc-4/jrc_mus-nacc-4.zarr/ but the actual multi-scale image is actually at s3://janelia-cosem-datasets/jrc_mus-nacc-4/jrc_mus-nacc-4.zarr/recon-2/em/fibsem-int16/. That requires a bit of guess work on the user side that we should be able to avoid. See here https://open.quiltdata.com/b/janelia-cosem-datasets/tree/jrc_mus-nacc-4/jrc_mus-nacc-4.zarr/