chore(generator): add compatability layer templates

[hebaalazzeh]

This PR consolidates the common code into a temporary compatibility layer as a fallback implementation as we move this code to google-api-core. Once, the minimum supported version of google-api-core has these functions we will clean up this layer.

Towards: #17883

[gemini-code-assist]

Code Review

This pull request introduces a compatibility module (_compat.py) and its corresponding unit tests (test_compat.py) to the GAPIC generator templates, updating the generator to allow these private templates and prevent them from being skipped when empty. The review feedback correctly identifies that prepending # before the license include statement in the template files results in double comment characters (# #) in the generated golden files, and suggests removing the leading # to fix this formatting issue.