Support dynamic change to DDL job configuration

[tangenta]

Feature Request

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

TiDB v8.3.0 removes the dynamic change feature for tidb_ddl_reorg_worker_cnt and tidb_ddl_reorg_batch_size, but this feature is useful in some scenarios:
The foreseeable business peak arrives during the DDL operation, and it is necessary to reduce DDL's impact on the cluster.

• DDL execution reaches a certain stage, and resource usage suddenly increases. The resources available to DDL must be reduced (for example, ADD INDEX entering the ingest stage usually incurs a large CPU memory overhead).
• During the DDL operation, the user observed that the data backfilling speed was slow, and it is necessary to increase the backfilling concurrency to complete the DDL within the expected time.

Although the above scenario can be bypassed by canceling and re-executing DDL, it reduces usability and affects user experience.

Describe the feature you'd like:

Add a new syntax that supports dynamically changing relevant config for DDL jobs.

ADMIN ALTER DDL JOBS *job_id* THREAD = 8;   -- tidb_ddl_reorg_worker_cnt
ADMIN ALTER DDL JOBS *job_id* BATCH_SIZE = 256;  -- tidb_ddl_reorg_batch_size
ADMIN ALTER DDL JOBS *job_id* MAX_WRITE_SPEED = '200MiB'; -- tidb_ddl_reorg_max_write_speed

This syntax refers to the ADMIN clause currently supported by TiDB.

ADMIN SHOW DDL JOBS ...
ADMIN PAUSE DDL JOBS ...
ADMIN RESUME DDL JOBS ...
ADMIN CANCEL DDL JOBS ...

THREAD and BATCH_SIZE only work for DDLs that need to reorganize data (reorg) types, currently including ADD INDEX, MODIFY COLUMN, and REORGANIZE PARTITION. Changing job config is not supported for non-reorg DDLs.

On the other hand, add a column COMMENTS to the result set of ADMIN SHOW DDL JOBS, which displays the current DDL job config:

ADMIN SHOW DDL JOBS *job_id*;

... |                 COMMENTS |
... | THREAD=4, BATCH_SIZE=256 |

Describe alternatives you've considered:

• Revert ddl: support session level tidb_ddl_reorg_worker_cnt and batch_size #55334

Teachability, Documentation, Adoption, Migration Strategy: