Google and generic OIDC provider sign-in

You can now sign in to Hive with Google or any generic OIDC provider. Configure HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET for Google, or HIVE_OIDC_ISSUER, HIVE_OIDC_CLIENT_ID, and HIVE_OIDC_CLIENT_SECRET for a custom provider. Each configured provider appears as its own button on the login page. Operators can also restrict access by email domain with HIVE_GOOGLE_ALLOWED_DOMAINS and HIVE_OIDC_ALLOWED_DOMAINS.