Add new system table "merge_tree_settings"

[kshvakov]

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[alexey-milovidov]

👍

A few notes:

• commits were squashed because there are commits from previous PRs; it's better to merge with upstream before PR;
• note that each table can have individual settings (SETTINGS clause in CREATE TABLE) and currently there is still no way to show (except SHOW CREATE TABLE) or change (except manual edit metadata files) these values.