Remove the HttpIntializer dependency from AuthCredentials

[aozarov]

After we resolve #334 We should also change AuthCredentials and replace protected abstract HttpRequestInitializer httpRequestInitializer with public abstract Credentials credentials() returning com.google.auth.Credentials.Credentials.

We should also change ServiceOptions.httpRequestInitializer to get the credentials from AuthCredentials and wrap them inside HttpCredentialsAdapter.

If we can't find a way to deal with the deferred scoping, we can consider returning GoogleCredentials
instead of credentials.

[aozarov]

After we are done with this issue I think we can change our version to 0.1 or higher.