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

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Drafted-by: GitHub Copilot (GPT-5.3-Codex)

Start by tracing the Google provider's existing GOOGLE_CLOUD_UNIVERSE_DOMAIN handling and the connection extras consumed by that provider. Add a per-connection universe-domain option that remains compatible with existing environment-variable usage, and verify that it is exposed through the Airflow connection UI and works for multiple connections.

Written by the indexing model from the issue text.