optimize_if_* session settings

These settings are available in system.settings and are autogenerated from the ClickHouse snapshot.

optimize_if_chain_to_multiif

Replace if(cond1, then1, if(cond2, …)) chains to multiIf. Currently it’s not beneficial for numeric types.

optimize_if_transform_strings_to_enum

Replaces string-type arguments in If and Transform to enum. Disabled by default cause it could make inconsistent change in distributed query that would lead to its fail.