`#[udf]` macro for simple functions

[tgross35]

It would be nice to provide a simplified interface for the most common needs:

#[udf]
fn my_udf(a: Option<&str>, b: f64, c: &str) -> Option<i32> {
}

#[udf]
fn my_udf2(cache: &mut Vec<u8>, a: Option<&str>, b: f64, c: &str) -> Result<&[u8], String> {
}

// If init is not specified, default to Default
#[udf(init = MyStruct::new())]
fn my_udf3(cache: &mut MyStruct, a: Option<&str>, b: f64, c: &str) -> &[u8] {
}

// desugars to something like
mod my_udf_mod {
    struct MyUdf(Vec<u8>);

    #[register]
    impl BasicUdf for MyUdf {
        returning = Option<i32>;
        fn init(...) { /* autogenerated, verify types */ }
        fn process(&mut self...) {
            // generate user defined types
            let arg1 = args.get(0).as_str();
            my_udf(self.0, arg1, arg2, arg3)
        }
    }
    
    fn my_udf(a: Option<&str>, b: f64, c: &str) -> Option<i32> {
    }
}

The work would be entirely in the proc macro and would just desugar to our current traits. Some way to define >1 function signature would also be cool, but I don't quite know how to combine them together without having them in the same block

[tgross35]

Actually... it may be easiest to add a derive argument for arg types

#[derive(SqlArgs)]
struct Args <'a>{
    a: u64,
    b: &'a str,
    c: Option<Vec<f64>>
}

#[udf]
fn foo( {a, b, c}: Args) -> String {
    todo!()
}