Don't do a full nested mount setup when listing a directory.

Currently, listing a directory, performs a filesystem setup for the directory and all of it's children, even though we don't really care about deeply nested mounts (see also https://github.com/nextcloud/server/issues/57350)

There are 2 ways we could optimize this.

1. Add an option to setup the filesystem for only 1 level of child-mounts instead of child-mounts at any depth.
2. Instead of setting up the actual mount points for the children, just fetch the metadata for the mountpoints (using oc_mounts) and then create LazyFolder stubs with them.

Option 2. should be the more optimal one, but it would require storing per-users permissions (or permissions mask) in oc_mounts to deal with the fact that different users have different permissions for shares/groupfolders.

Start by tracing the directory-listing filesystem setup and how child mounts are represented in oc_mounts and LazyFolder. Compare the two optimization options described in the issue, including their implications for per-user permissions. The issue does not define a chosen approach or concrete completion criteria, so those need to be clarified before implementation.