This updates the credential store resources to use a oneof for
attributes. This allows requests to the credential store service to
leverage the new subtypes.AttributeTransformerInterceptor so that
attributes are strongly typed prior to the service handler.
This moves the generated pb files to the `sdk` module. This allows third parties to make use of them if desired (e.g. in upcoming plugins) and moves away from the `gen` nomenclature in the directory, since we are going to be adding some extra interface functions to the messages.
Timothy Messier
Jeff Mitchell
Todd Knight