feat(core): implement PEP 0810 explicit lazy imports in google-cloud-core

[hebaalazzeh]

This PR implements PEP 0810 explicit lazy imports in google-cloud-core.

On Python 3.15+, this defers loading of heavy inner modules and third-party dependencies (grpcio, cryptography, requests, and protobuf descriptor pools) to reduce serverless cold starts and memory footprints.

On Python 3.14 and below, this falls back safely to eager execution with zero backwards-compatibility risk.

Related Links

Design doc: go/sdk:python-lazy-loading

[gemini-code-assist]

Code Review

This pull request implements PEP 0810 explicit lazy imports (lazy_modules) across several modules in google-cloud-core to optimize cold starts and memory footprints in Python 3.15+. The review feedback correctly identifies that using importlib.util.find_spec on submodules (such as google.auth.api_key) eagerly imports their parent packages, which defeats the purpose of lazy loading. The reviewer suggests using module-level getattr to defer the resolution of HAS_GOOGLE_AUTH_API_KEY until it is actually accessed.