codegen/function: body future handles ownership of slice argument

[BiagioFesta]

Currently, the future body generated is something like:

pub fn probe_endpoints_future(
    my_slice: &[MyObject],
) -> Pin<
    Box_<dyn std::future::Future<Output = Result<_, glib::Error>> + 'static>,
> {
    skip_assert_initialized!();

   let my_slice = my_slice.map(ToOwned::to_owned);   // if my_slice is declared as nullable
             // or
   let my_slice = my_slice.clone();                  // if my_slice is not declared as nullable

    Box_::pin(gio::GioFuture::new(&(), move |_obj, cancellable, send| {
      // ...
}

both approaches fail to compile as:

• .map does not exist for slice type (it is not an Option)
• .clone() does not really give ownership of the data slice, as clone only copies the wide-pointer.
  • This is an issue as the future closure need to move something which needs 'static lifetime (and that's the reason why the code in the generator performs a clone or to_owned in the first place).

This PR aims to handle the case of slice argument

[BiagioFesta]

CI is failing, but I don't think it is related with the changes:

Resolving github.com (github.com)... 140.82.116.4
Connecting to github.com (github.com)|140.82.116.4|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2024-06-12 15:30:35 ERROR 404: Not Found