Explore s3 access structure

[tcompa]

There are a few ongoing discussions where we would benefit from a better understanding of "paths" in s3 object-storage. Examples are the definition of components and the definition of Dataset attributes.

This issue is very open-ended, but the goal is to have something like:

• Establish read/write access to a Zarr array/group from s3 by providing its full URL
• Explore options for providing the equivalent of parent-folder URLs and then relative paths to a specific group or array
• Compare to the equivalent operations for local filesystem storage

[tcompa]

This issue is not currently in-progress, since most of the basic tests were already performed in https://github.com/fractal-analytics-platform (and the definition of components already took place, with fractal-server v2). Closing.