Currently prepare_expression is never used where a schema is needed -
it is called for the right-hand-side of binary operators (where we
don't accept columns) or for attributes like WRITETIME or TTL. But
when we unify expression preparation it will need to handle columns
too, and these need the schema to look up the column.
So pass the schema as a parameter. It is optional (a pointer) since
not all contexts will have a schema (for example CREATE AGGREGATE).
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