Automatically switch from FUSE mount to xrootd for EOS sites

[nsmith-]

In https://root.cern/doc/v628/release-notes.html#faster-reading-from-eos ROOT has used a local file's xattr to determine if an xrootd endpoint can provide the file. This is generally preferred over FUSE mounts. Since this is a bit of a niche use case, it seems best to keep it in uproot rather than lower-level libraries such as fsspec that would otherwise not want to waste time probing xattr when most of their customers do not use EOS. In python, I can find both xattr and pyxattr packages to query these, but I am not sure if we want to take that dependency or not.
cc @sciaba