Introduce `Expr` variants for `IS [NOT] TRUE / FALSE / UNKNOWN`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
In #3252 we added support for SQL IS [NOT] TRUE/FALSE by mapping to the existing logical binary operator Is[Not]DistinctFrom. This resulted in a small PR compared to introducing new Expr variants such as IsNotTrue, but the downside is that we are now unable to implement type validation/coercion to ensure that the left expression is a boolean value.

Describe the solution you'd like
Introduce new Expr variants but still map to the physical plan for IsDistinctFrom.

Describe alternatives you've considered
None

Additional context
None

[andygrove]

@sarahyurick fyi

[andygrove]

Another option would be to add Operatror::IsTrue instead of a new Expr

Never mind, IS TRUE is a unary expression, not binary.

[liukun4515]

Another option would be to add Operatror::IsTrue instead of a new Expr

Never mind, IS TRUE is a unary expression, not binary.

Hi @andygrove Do we need the other unary expression for IS_TRUE, IS_FALSE... ？
I go through preview pr for adding the sql syntax for above expr like that which convert the IS_TRUE to the binary expr

SQLExpr::IsNotTrue(expr) => Ok(Expr::BinaryExpr {
                left: Box::new(self.sql_expr_to_logical_expr(*expr, schema, ctes)?),
                op: Operator::IsDistinctFrom,
                right: Box::new(lit(true)),
            }),

I am not sure if it is a good way.