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`.

Start with google/cloud/storage/oauth2/credentials.h, google/cloud/storage/client.cc, and google/cloud/storage/client_options.cc, especially the cited credential mapping. Trace how storage credentials and google::cloud::Credentials flow through the storage internals. Done means the implementation uses GUAC credentials and the mapping direction is from storage credentials to GUAC, without relying on the deprecated public credential classes.

Written by the indexing model from the issue text.