Consider add another way to support traits

[taiki-e]

Currently, we can use proc-macro-derive to extend the supported traits, but we can consider providing another way.

[taiki-e]

ambassador is interesting. Maybe we can support something like the following:

#[delegatable_trait]
pub trait Trait {
    // ...
}

#[auto_enum(delegate(Trait))]
fn func() -> impl Trait {
    // ...
}

generate:

fn func() -> impl Trait {
    #[derive(::ambassador::Delegate)]
    #[delegate(Trait)]
    enum __Enum1<__T1, __T2> {
        __V1(__T1),
        __V2(__T2)
    }

    // ...
}