*: add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log

[zimulala]

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pingcap/tidb#64010
Add slow log control variables (tidb_slow_log_rules, tidb_slow_log_max_per_sec) and hint write_slow_log

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@zimulala: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	a16078e	link	true	/test pull-verify

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[zimulala]

其中峰值可能超过配值（峰值 = 2*tidb_slow_log_max_per_sec），平均打印速率仍然受 tidb_slow_log_max_per_sec 限制。这个内容是否需要说明？