(Is there already) support for async stream

Hi, I also commented on #507 as it seems related (at least to someone not too familiar with S/R yet).

A little bit of context of what I want: I am looking for something that represents an async stream, to sketch out an example:

```C++
unifex::task getFooSum(/* some params here */)
{
auto request = /* build request from params */

// This range of responses is filled from the underlying RPC framework, responses are arriving on
// a separate thread (what we've termed "the callback thread" internally.
auto rangeOfMultipartResponses = someRpcClient->sendRequest(request);
auto fooValueSum = rangeOfMultipartResponses
unifex::reduce_stream(0, [](auto resp){ resp.fooValue(); });
co_return co_await fooValueSum;
}
```

Is this something that the generic `range_stream` would support OR we need to build our own stream adapter? And isn't this something generic enough to be present in `libunifex`? Basically something like `cppcoro::async_generator` that speaks the stream protocol? I have seen `cppcoro::async_generator` being mentioned in the docs, but wasn't able to find anything that can support that functionality here.