sys-var: add SESSION scope for ddl_reorg_batch_size and ddl_reorg_worker_cnt

[hfxsd]

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• This PR is translated from:sys-var: add SESSION scope for ddl_reorg_batch_size and ddl_reorg_worker_cnt docs-cn#18359
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[lilin90]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-14 03:46:29.600141798 +0000 UTC m=+326074.303611442: ☑️ agreed by tangenta.
• 2024-08-14 06:22:19.153679095 +0000 UTC m=+335423.857148735: ☑️ agreed by lilin90.

[hfxsd]

/approve

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