TableExpression contains a Table reference to its corresponding ITableBase in the relational model; we can do the same with ColumnExpression. This would e.g. allow translation to easily look at the relational model when doing decision making. For example, npgsql/efcore.pg#3546 is about generating different SQL based on whether an index is defined on a column - this would enable that.

Note that unlike TableExpression.Table, ColumnExpression.Column would be nullable, as some ColumnExpressions have no associated column (e.g. subquery projections).