Add command groupings to vtctldclient

[ajm188]

Feature Description

The old vtctlclient would group commands nicely into categories in the help output, e.g.:

Serving Graph:
  GetSrvKeyspaceNames <cell>
  GetSrvKeyspace <cell> <keyspace>
  GetSrvVSchema <cell>
  DeleteSrvVSchema <cell>

Replication Graph:
  GetShardReplication <cell> <keyspace/shard>

Workflow:
  VExec <ks.workflow> <query> --dry-run

Workflow:
  Workflow <ks.workflow> <action> --dry-run

Cells:
  AddCellInfo [-server_address <addr>] [-root <root>] <cell>
  UpdateCellInfo [-server_address <addr>] [-root <root>] <cell>
  DeleteCellInfo [-force] <cell>
  GetCellInfoNames 
  GetCellInfo <cell>

It would be nice to have this sort of grouping in the vtctldclient as well. There's an open issue on cobra for this, which itself links to a PR on google/kf, which might be some useful references in how we could do this as well.