schemeshard: action to move tablet to another storage pool (#45964)

New schemeshard admin action `MoveToStoragePool` moves a shard to another storage pool.

- Button on the shard-info page
- Target storage pool is picked from the database's storage pools
- Moving a shard is really a rebinding of all the tablet's channels to the target storage pool
- Done by sending `CreateTablet` with the updated channel bindings to the Hive
- New bindings are persisted on the Hive's ack (`CreateTabletResult`)
- Failure to get an ack leaves nothing persisted on the schemeshard side
- So the action can be retried
- `CreateTablet` is idempotent — sending it more than once does no harm
- Still, moving a shard to the storage pool it's already on is a no-op
- Unless the confirmation box (next to the button) is checked to force the send to the Hive

The action is universal — works for any subdomain's shard/tablet.
On a tenant schemeshard, a shard-info page can show empty channel bindings: those shards are the tenant's system tablets (their channel bindings live only on the root schemeshard). In that case there's no button (the move is rejected); the UI links to the root schemeshard page instead, with an explanation.

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