Support direct shard COPY

The process utility hook gives us some liberty to deal with non-existent names in utility commands such as COPY.

That means we could support commands that copy directly to/from shards via the coordinator, e.g.:
```sql
COPY data_102008 FROM STDIN;
COPY data_102008 TO STDOUT;
```

If the table does not exist, the coordinator could check whether it matches a shard name and redirect the COPY to the shard.

This would be helpful for migrations to and from Citus.