sysvars: add tidb_shard_row_id_bits and tidb_pre_split_regions

[jackysp]

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[lilin90]

/lgtm

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Timeline:

• 2024-09-24 03:09:57.983190677 +0000 UTC m=+1535467.723614611: ☑️ agreed by hfxsd.
• 2024-10-08 09:08:58.620551251 +0000 UTC m=+950694.040764263: ☑️ agreed by lilin90.

[hfxsd]

/approve

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