Consider adding a `recursive` argument to `ls()`

[hagenw]

As shown in audeering/audb#458 for backends that support folder structures (like Minio and Artifactory), calling our current implementation of ls() to get the first layer of folders can be very expensive.
We could consider adding a recursive=True argument to ls() to not break the current behavior. A user could then set it to recursive=False, and backends that support it, could then return the first layer, whereas backends that don't support folders return the same as for recursive=True.