Clean up shared helpers

[daniel-sanche]

We have some helper functions like _use_client_cert_effective, which are repeatedly generated across files. We should try to consolidate them into shared code

Source:

Each service will have this function repeated which can cause bloat in the generated code when there are many services.

Originally posted by @parthea in googleapis/gapic-generator-python#2472 (comment)

[parthea]

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