FUSE: add sharding support for top-level dirs

The archive/ and meta/ subdir of the swh-fuse mountpoint might grow quite big.

We should add (optional?) sharding supports for them, so that entries there can be created according to a namespace like ab/cd/swh:1:XYZ:abcd.... The depth should be configurable, and 1 (i.e., /ab/ only) could be a good default.

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Migrated from T2694 (view on Phabricator)