Async Client Default Options

**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 to bring the Async Client Default Options in line with the gRPC client default options. For example, we should be using direct path when we can detect we are running within GCP.

Start by comparing the GCS Async Client default options with the gRPC client default options, focusing on how direct path is selected when running within GCP. Done means the async defaults are brought in line with the gRPC defaults, including direct-path use when GCP can be detected.

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