feat: A system table for showing the parallelism of streaming jobs

[neverchanje]

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

We currently need to join several low-level system tables rw_parallel_units, rw_materilaized_views and rw_sinks to get a final view for parallelisms.

Describe the solution you'd like

We should provide a view called rw_streaming_parallelism for this purpose. In addition to parallelism number, we also need a column indicating the scaling policy of the object.

Describe alternatives you've considered

WITH all_objects AS (
    SELECT id, name FROM rw_tables
    UNION ALL
    SELECT id, name FROM rw_materialized_views
    UNION ALL
    SELECT id, name FROM rw_sinks
    UNION ALL
    SELECT id, name FROM rw_indexes
),
fragment_parallelism AS (
    SELECT
    f.fragment_id,
    COUNT(a.actor_id) AS parallelism
    FROM rw_actors a
    INNER JOIN rw_fragments f ON a.fragment_id = f.fragment_id
    GROUP BY f.fragment_id
)
SELECT
    f.*,
    fp.parallelism
FROM all_objects ao
INNER JOIN rw_fragments f ON ao.id = f.table_id
INNER JOIN fragment_parallelism fp ON f.fragment_id = fp.fragment_id
ORDER BY ao.name;

Here is the query I used to write for our previous scaling manual. https://docs.risingwave.com/docs/current/k8s-cluster-scaling/

Additional context

No response