Promote the GCS+gRPC plugin to GA

[coryan]

This change is more involved than I thought at first, so I am creating a bug. We want to declare GA for the GCS+gRPC plugin. This is basically one new public function, but we also want to:

• The storage_grpc library is part of __ga_libraries__
• The target names with CMake are google-cloud-cpp::storage_grpc and google-cloud-cpp::storage_grpc_mocks
• The rule names with Bazel are //:storage_grpc and //:storage_grpc_mocks.
• The target names and rules names with the experimental- prefix should be preserved for at least one release.
• Manually setting GOOGLE_CLOUD_CPP_ENABLE_CTYPE_CORD_WORKAROUND is tedious, we should let CMake figure this out.
• We need to make a number of small tweaks to the ci/ scripts and their support files. May be cleaner to make them as separate PRs.