planner: Add new variable named "tidb_opt_selectivity_factor"

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What is changed, added or deleted? (Required)

Add new variable named "tidb_opt_skew_distinct_agg" and explain it

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What is the related PR or file link(s)?

• This PR is translated from: planner: Add new variable named "tidb_opt_selectivity_factor" docs#21655
• Other reference link(s): planner,selectivity: record the default selectivity tidb#63102

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• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[elsa0520]

Suggested change

	- 该变量用于指定 TiDB 优化器的伪选择率。在某些情况下，当优化器无法根据统计信息推导出谓词选择率时，优化器将使用此伪选择率作为替代值。**不建议**修改此值。
	- 该变量用于指定 TiDB 优化器的默认选择率。在某些情况下，当优化器无法根据统计信息推导出谓词选择率时，优化器将使用此默认选择率作为替代值。**不建议**修改此值。