Create a public API to get access token

[coryan]

The APIs to get (and cache) access tokens are all in *internal* namespaces. We need a simple API to create access tokens.

Maybe something like this:

namespace google::cloud {
struct AccessToken { std::string token; std::chrono::time_point expiration; };

class AccessTokenGenerator {
 public:
   virtual ~AccessTokenGenerator() = default;
   virtual StatusOr<AccessToken> GetToken() = 0;
};

std::shared_ptr<AccessTokenGenerator> MakeAccessTokenGenerator(std::shared_ptr<Credentials>);

That is a fairly minimal API, whatever our implementation details we will always have enough to support it.

We need to think about the name of the libraries too. All of this lives in rest_internal which suggests it is not a public library. We may need to create google_cloud_cpp_auth ?