Cleaner API to create Expr::ScalarFunction programatically

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I am trying to programatically create Exprs that represent function calls, for example to_timetsamp(x, 4) or something

To do so today you have to do something like this (from simplify_expressions.rs) which is 🤮

        // to_timestamp("2020-09-08T12:00:00+00:00")
        let expr = Expr::ScalarFunction {
            args: vec![lit("2020-09-08T12:00:00+00:00")],
            fun: BuiltinScalarFunction::ToTimestamp,
        };

Describe the solution you'd like
I would like to write code like this

        // to_timestamp("2020-09-08T12:00:00+00:00") --> timestamp(1599566400000000000i64)
        let expr = call("to_timestamp", vec![lit("2020-09-08T12:00:00+00:00")])
          .unwrap();
```rust

Perhap with an api like this

```rust
/// Calls a named built in function
///
/// ```
/// use datafusion::logical_plan::*;
///
/// // create the expression sin(x) < 0.2
/// let expr = call("sin", vec![col("x")]).unwrap().lt(lit(0.2));
///
/// ```
pub fn call(name: impl AsRef<str>, args: Vec<Expr>) -> Result<Expr> {
...
}

Note you can lookup a str to scalar function by name with something like:

    let fun = name.as_ref().parse::<BuiltinScalarFunction>()?;

Additional context
There are a bunch of this nonsense in simplify_expression.rs which could be cleaned up

[mkmik]

What about something like:

call_builtin_scalar_fn!(ToTimestamp, lit("2020-09-08T12:00:00+00:00"))

[alamb]

call_builtin_scalar_fn!(ToTimestamp, lit("2020-09-08T12:00:00+00:00"))

That would have the upside that the compiler could check the name

It has the downside that the user has to know the mapping from the sql level name (e.g. concat to Concat) which maybe isn't a big deal.

Perhaps we could do both 🤔

[HaoYang670]

I'd like to have a try if no one has been doing it😀

[mkmik]

@HaoYang670 thanks

[HaoYang670]

I find some duplicate tests in simplify_expression.rs.
Tracked in issue #1727