Add support for autosharding

[alrevuelta]

Add support for autosharding, suggesting 8 shards like The Waku Network.

  --pubsub-topic=/waku/2/rs/99/0
  --pubsub-topic=/waku/2/rs/99/1
  --pubsub-topic=/waku/2/rs/99/2
  --pubsub-topic=/waku/2/rs/99/3
  --pubsub-topic=/waku/2/rs/99/4
  --pubsub-topic=/waku/2/rs/99/5
  --pubsub-topic=/waku/2/rs/99/6
  --pubsub-topic=/waku/2/rs/99/7

--cluster-id=99