Support multiple ZARR_DATA_BASE_PATHs

[tcompa]

If we want to proceed with fractal-analytics-platform/fractal-web#527, and meanwhile also keep the possibility of mounting other shares (apart from the main one, which will be mounted under the same local path as the remote one), then it would be useful to accept multiple ZARR_DATA_BASE_PATH values.

Side note: right now, we can provide either an absolute path (which should be within ZARR_DATA_BASE_PATH) or a relative one (which will be made absolute by using ZARR_DATA_BASE_PATH as a base path). If we support multiple values for ZARR_DATA_BASE_PATH, then we'll always have to provide an absolute path. This is clearly not a big deal, especially if the URL is created within fractal-web (as in fractal-analytics-platform/fractal-web#527).

[tcompa]

Note that what this issue (together with fractal-analytics-platform/fractal-web#527 and https://github.com/fractal-analytics-platform/fractal-deployments/issues/16) suggests that we'll need to review strategies for mapping remote paths to local ones. This is not a critical issue for the current exploratory phase of fractal-vizarr-viewer, but we'll eventually have to settle for some option (and we'll have to decide where this option is implemented, together with any kind of authorization rule).

[tcompa]

For the specific use case I had in mind, this is not so relevant.
We can have ZARR_DATA_BASE_PATH=/something/, then mounted zarrs are in /something/myuser/x.zarr but we can still place some testing folder in /something/else/.