Add conditional logic in `_compat.py.j2` to import static code from google-api-core

As part of refactoring the generator we are moving static code within templates to `_compat.py.j2` and google-api-core. Add conditional logic to check if this static code exists in google-api-core and use it from there, if not then use the fallback implementation within the file.

Start by reading the generator template `_compat.py.j2` and locating the static code that is being moved. Then inspect how google-api-core exposes that code and how the generator handles conditional imports. Done means the template uses the google-api-core implementation when available and retains its fallback when it is not.

Scritto dal modello di indicizzazione a partire dal testo della issue.