Support helper class

[dizcology]

Some libraries support thin handwritten helpers/mixins, and currently this may depend on owlbot.py postprocessing to update the generated code. The GAPIC generator perhaps should become aware of the existence of these helpers/mixins and generate the code accordingly.

Example: python-speech

Handwritten code: https://github.com/googleapis/python-speech/blob/09cf275de920d169c26b8ed364d9e3abb18f8781/google/cloud/speech_v1/helpers.py Updated generated code: https://github.com/googleapis/python-speech/blob/09cf275de920d169c26b8ed364d9e3abb18f8781/google/cloud/speech_v1/init.py#L61-L65 owlbot.py: https://github.com/googleapis/python-speech/blob/09cf275de920d169c26b8ed364d9e3abb18f8781/owlbot.py#L39-L66