Add a `pylint` presubmit check on the golden files

[parthea]

I ran pylint on the generated python client for google-cloud-optimization locally and received several warnings. See the attached build log. Several of these warnings appear to be valid issues. We should address the warnings and add a pylint presubmit check to ensure that the pylint continues to succeed once issues are addressed.

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.