This interface allows legacy classes to be injected by
ConfigurationBuilderImpl. To be of any use, implementations of this class
must have at least one constructor with an @Inject annotation. From
ConfigurationBuilderImpl's perspective, an ExternalConstructor class is just
a special instance of the class T, except that, after injection an
ExternalConstructor, ConfigurationBuilderImpl will call newInstance, and
store the resulting object. It will then discard the ExternalConstructor.