ConfigT- A configuration object which will be populated with the external configuration.
InputT- The input type of the externally configured PTransform.
OutputT- The output type of the externally configured PTransform.
Classes which implement this interface will be instantiated externally and require a zero-args
buildExternal method will be called with the configuration object as a
This builder needs to be registered alongside with a URN through
ExternalTransformRegistrar. Note that the configuration requires setters for all configuration
parameters, e.g. if there is a parameter "start", there should be a corresponding setter