Refactor storage credentials

[dbolduc]

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

google-cloud-cpp/google/cloud/storage/oauth2/credentials.h

Line 45 in 80a518a

* @deprecated Prefer using the unified credentials documented in @ref guac

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

google-cloud-cpp/google/cloud/storage/client.cc

Line 50 in 80a518a

internal::DefaultOptionsWithCredentials(std::move(opts))) {}

We map GUAC -> storage credentials.

google-cloud-cpp/google/cloud/storage/client_options.cc

Lines 296 to 300 in 80a518a

	if (opts.has<UnifiedCredentialsOption>()) {
	auto credentials =
	internal::MapCredentials(*opts.get<UnifiedCredentialsOption>());
	return internal::DefaultOptions(std::move(credentials), std::move(opts));
	}

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.

[dbolduc]

For the record, we do not need to refactor. We can "delete" the storage::oauth2 classes by moving them into storage_internal and keeping the implementation mostly the same.

We might do this to unblock the next major release.

It is just that the resulting implementation would be unnecessarily complicated compared with using GUAC types.