batch: support for setting scan parallelism

[zwang28]

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

The current batch_parallelism doesn't affect batch scan parallelism. The batch scan parallelism always equals to the stream parallelism of the table.
It's not always proper to change stream parallelism in order to get an ideal batch scan parallelism. Some downsides are:

• it brings more memory footprints in compute node.
• it results larger checkpoint size which impair storage compaction performance.

Describe the solution you'd like

Add a session variable batch_scan_parallelism. If set, it takes precedence.

Describe alternatives you've considered

No response

Additional context

No response

[zwang28]

Just don't limit max batch scan parallelism by stream parallelism.

risingwave/src/meta/src/serving/mod.rs

Line 55 in 1f4b882

// Set max serving parallelism to `streaming_parallelism`. It's not a must.