Expose a shard range calculator as a vtctl command

[ajm188]

Feature Description

vtctl should expose a new command, which, given a number of total shards in a keyspace, outputs a list of the hex shard ranges for a keyspace of that size.

Example usage:

❯ vtctlclient ListShardRanges -num_shards 2
[
  "-80",
  "80-"
]

Use Case(s)

From the vitess slack I've seen at least one other person maintaining a tool like this, and I would imagine that other users of vitess have their own solution to this problem. Providing this in vitess removes an additional tool for all those users to maintain, and guarantees that we're all using the same logic.