a status flag for scaling

[neverchanje]

Is your feature request related to a problem? Please describe.

Only the RisingWave kernel is able to tell whether the scaling is still in progress.
Therefore, we should expose a flag in a system table indicating the scaling status.
With the flag, RW Cloud can collect the start time and end time of scaling by polling for the status change.

Describe the solution you'd like

I'd like to propose a system table rw_cluster_info:

| instype | address | version | githash | status | starttime | uptime |

Once all nodes are 'ready', the cluster can be viewed as recovered from downtime.

--- It's supposed to return an empty result set.
SELECT *
FROM rw_cluster_info
WHERE status != 'ready';

The status column is redundant from an informational completeness perspective and is maintained only for clarity. The uptime column already indicates whether the cluster is ready. If a node is dead, the uptime field should be set to null.

@shanicky Please correct me if my understanding is different from the implementation.

Describe alternatives you've considered

No response

Additional context

also cc: @xuefengze