Add absl::Cord to storage/async/write_payload.h constructors

[laramiel]

What component of google-cloud-cpp is this feature request for?
google/cloud/storage async api

Is your feature request related to a problem? Please describe.
I have absl::Cord data to write to GCS; the current api uses absl::Cord internally but does not allow passing a Cord to the call.

Describe the solution you'd like
Add the following constructor:

  explicit WritePayload(absl::Cord p) : impl_(std::move(p)) {}