Add support for snowflake cluster by expressions

[osipovartem]

CLUSTER BY (to_date(ts), coalesce(foo, 'bar'))

Failed with ParserError("Expected: ), found: (
https://docs.snowflake.com/en/user-guide/tables-clustering-keys#defining-a-clustering-key-for-a-table

We need to extend/change CreateTable cluster_by type from

pub cluster_by: Option<WrappedCollection<Vec<Ident>>>,

to

pub cluster_by: Option<WrappedCollection<Vec<Expr>>>,

This allows statements like CLUSTER BY (to_date(start_tstamp)) to be parsed correctly.