What component of google-cloud-cpp is this feature request for? For
GCS

Is your feature request related to a problem? Please describe. A clear and
We need a function to convert a storage_experimental::AsyncReader into a single future<StatusOr>. The payload should be the concatenation of all the payloads returned (asynchronously) by AsyncReader.
We already have something that is close:

https://github.com/googleapis/cpp-storage-prelaunch/blob/54274c0fb947883b0dce63584dee8481d5c26cc0/google/cloud/storage/internal/async/connection_impl.cc#L212-L218

Maybe we just need to add the function to a public header, add some documentation and so forth.