Google provider: support universe_domain via connection extras

### Body The current Google provider universe-domain support relies on the process-wide environment variable GOOGLE_CLOUD_UNIVERSE_DOMAIN (introduced in https://github.com/apache/airflow/pull/66159). That is constraining for real deployments: - A single Airflow deployment cannot target multiple universes at once. - The configuration is not visible/discoverable in the Airflow connection UI. This issue tracks follow-up work to support universe domain configuration through Google connection extras (with a compatibility path for existing env-var usage). Related review comment: - https://github.com/apache/airflow/pull/66159#discussion_r3170370794 --- Drafted-by: GitHub Copilot (GPT-5.3-Codex)