Enable WasmStr/WasmList with call_concurrent #12467
Description
Right now it's not possible to use these types as return values of a guest function with call_concurrent. The reason for this is that post-return is invoked automatically meaning that using these types would be using already-free'd memory in the guest. We should enable these types to work to unlock the possibility of zero-copy return values. This'll probably look like some sort of closure/trait passed to call_concurrent (ish) which is invoked once the values are received.
This is somewhat related to #12427 insofar as post-return is causing issues there.