support `Statement::ShowVariable` to show session configs

[waitingkuo]

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i'd like to implement Statement::ShowVariable to show session configs.
#3255 depends on this as well since we need SHOW TIMEZONE; to show current session time zone

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It'll be similar as https://www.postgresql.org/docs/current/sql-show.html

I plan to add a InformationSchemaSettingsBuilder that collects session configs from SessionState

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