Add an option for enabling/disabling/configuring the /zarrs/ hierarchy

[jwodder]

/zarrs/ is unlikely to be of use to people pointing a dandidav instance at their own Archive (unless they maintain their own set of Zarr manifests and edit dandidav's code to use it), and hence it should be possible to disable it.

Alternatively, /zarrs/ could be disabled by default, and enabling it could require specifying the manifest root URL and S3 download prefix URLs on the command line.

This will likely be easier to implement after #67 is implemented.