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 https://github.com/googleapis/gapic-generator-python/pull/2472#discussion_r2519605871