Dynamically resize / split shards if they grow too large

We had several requests from prospective and current customers about dynamically resizing their shards. This for example could become handy if a customer starts with 32 shards, expands their cluster size (# of machines), and then wants to double the shard count to 64 to benefit from a larger cluster.

With tenant isolation (#844), we introduced new primitives to split a shard into smaller ones and rebalance those shards to new machines. We could use those primitives to enable shard resizing / splitting functionality, not only for tenants but for dynamic shard resizing as well.