Allow flatten on arguments

[ShaddyDC]

Description of the feature

I've seen the flatten attribute in a bunch of places, but I don't think it's possible to add it to arguments yet. It would be useful to compose some structures better while keeping the API simple.
Let me know if it's already a thing and I missed it.

Code example (if possible)

#[derive(InputObject)]
struct MyData {
    a: String,
    b: String,
}

impl MyData {
    fn compute_hash(&self) -> String {
        let Self { a, b } = self;
        format!("{a}-{b}")
    }
}

#[Object]
impl QueryRoot {
    async fn test_1(&self, _ctx: &Context<'_>, #[graphql(flatten)] data: MyData) -> String {
        data.compute_hash()
    }
    async fn test_2(&self, _ctx: &Context<'_>, a: String, b: String) -> String {
        let data = MyData { a, b };
        data.compute_hash()
    }
}

Basically, I would like test_1 and test_2 to have the same calling interface, especially in cases where I've only got a single argument.
In simple cases like this, it's simple enough to just construct MyData inside the function, but for more complicated cases, it gets very verbose, and it also gets weird when you may have some copyable variables both inside MyData and also as a plain function argument floating around.

[carlocorradini]

This is super useful! 🤯🥳