Executor: dynamically changing the concurrency of the Scan operator

[fzhedu]

Feature Request

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

#18444, after increasing the concurrency of operators but decreasing the concurrency of Scan, the OOM disappears.

Describe the feature you'd like:

dynamically channging the concurrency of the scan operator, because the default 15 concurrency would buffer so many regions that occupy a lot of memory. This feature would avoid OOM in many cases.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[SunRunAway]

Does #17324 fix this issue?

[fzhedu]

Does #17324 fix this issue?

no. I planned let the PR fix this issue, but let it as the TODO work. It should not open too many copWorkers at the beginning.