feat: struct level #[difference(recurse)] #42

Allow #[difference(recurse)] to be used at the struct level.
The latter allows to recursively apply recurse to all struct fields without having to provide #[difference(recurse)] for each one.
Furthermore, if a field should skip the struct level recurse (i.e., an unit enum), allow #[difference(recurse(skip))].

#[derive(Difference, Debug, Clone)]
#[difference(recurse)]
pub struct Entity {
    pub id: String,
    #[difference(recurse(skip))]
    pub status: MyEnum,
    pub location: MyStruct,
}