Support column iterator for PartialModelTrait

[belovaf]

Sometimes I want to select a PartialModel DTO with complex query.
It would be very convenient if I could provide model columns to Query::select().columns(...).
Currently I'm using something like this:

#[derive(Debug, Default)]
struct ColumnsCollector {
    vec: Vec<ColumnRef>,
}

impl<'a> SelectColumns for &'a mut ColumnsCollector {
    fn select_column<C: ColumnTrait>(self, col: C) -> Self {
        let (t, c) = col.as_column_ref();
        self.vec.push(ColumnRef::Column(c));
        self
    }
}

But I believe that it should be a part of sea-orm.