C++ coroutines support

I would like to propose the integration of native C++20 coroutine support into the google-cloud-cpp client libraries, with a particular focus on Google Cloud Storage (GCS).

While the current asynchronous API is functional, the addition of first-class coroutine support (e.g., ``Task`` or ``Awaitable`` types) would significantly improve code readability and maintainability for modern C++ backends. Furthermore, providing low-level networking primitives—similar to the models found in ``boost::asio`` or the Networking TS—would allow developers to integrate GCS operations into their own custom execution contexts and coroutine frameworks.