[feat] Support scaling

[ysqyang]

The current implementation does not support horizontal scaling (scale-out or scale-in).

When increasing the number of shards, new pods are created, but there is no slot migration or rebalancing logic to redistribute slots from existing shards to the new ones. As a result, the new shards remain unused.

Similarly, decreasing the number of shards is currently a no-op, with no data or slot reassignment performed.