Refactor storage credentials

Storage introduced credentials in the public namespace `google::cloud::storage::oauth2`. These credential classes are now deprecated in favor of GUAC (`google::cloud::Credentials`):

https://github.com/googleapis/google-cloud-cpp/blob/80a518a357c93eec7ddc49666efcd7984135dafb/google/cloud/storage/oauth2/credentials.h#L45

However, the storage internals are implemented in terms of `google::cloud::storage::oauth2::Credentials`.

https://github.com/googleapis/google-cloud-cpp/blob/80a518a357c93eec7ddc49666efcd7984135dafb/google/cloud/storage/client.cc#L50

We map GUAC -> storage credentials.

https://github.com/googleapis/google-cloud-cpp/blob/80a518a357c93eec7ddc49666efcd7984135dafb/google/cloud/storage/client_options.cc#L296-L300

This is backwards. We need to map storage credentials to GUAC and use GUAC in the implementation, if we ever want to remove the deprecated credentials in `google::cloud::storage::oauth2`.