Add vtctld get-shard command

[maxenglander]

Adds a hidden vtctld command to read live shard records from the cluster:

pscale branch vtctld get-shard <database> <branch> --keyspace <keyspace> --shard <shard>

Depends on planetscale/planetscale-go#314.

Manual testing

Tested end-to-end against a dev Vitess branch (org with direct_vtctld_access enabled) with the full stack deployed.

1. Unsharded keyspace:

   pscale branch vtctld get-shard <database> <branch> --keyspace <keyspace> --shard -
   

   returned:

   {
     "keyspace": "<keyspace>",
     "name": "-",
     "key_range": {
       "start": "",
       "end": ""
     },
     "tablet_controls": [],
     "is_primary_serving": true
   }

2. Created a sharded keyspace with --shards 2:

   pscale keyspace create <database> <branch> sharded-ks --cluster-size PS-10 --shards 2 --wait
   

3. pscale branch vtctld get-shard <database> <branch> --keyspace sharded-ks --shard -80 returned:

   {
     "keyspace": "sharded-ks",
     "name": "-80",
     "key_range": {
       "start": "",
       "end": "gA=="
     },
     "tablet_controls": [],
     "is_primary_serving": true
   }

4. pscale branch vtctld get-shard <database> <branch> --keyspace sharded-ks --shard 80- returned:

   {
     "keyspace": "sharded-ks",
     "name": "80-",
     "key_range": {
       "start": "gA==",
       "end": ""
     },
     "tablet_controls": [],
     "is_primary_serving": true
   }

key_range.start/end are base64-encoded bytes (Vitess protojson encoding); "gA==" is byte 0x80, the split point between the two shards.