Add command to manually split a shard

Because data distribution does not currently take into account read load, we could end up with a small read-hot key range that is never split into multiple shards. If we could manually insert a shard boundary through the management API, we could split these key ranges across multiple shards, even if the created shards are smaller than `MIN_SHARD_BYTES`.

Start with the management API and the existing shard-splitting behavior, paying attention to the MIN_SHARD_BYTES constraint. Determine how a manually inserted shard boundary should be represented and validated; done means read-hot key ranges can be split through the API even when the resulting shards are smaller than MIN_SHARD_BYTES.

Written by the indexing model from the issue text.