Sign in with Google, generic OIDC, or both

Sign in through Google, a generic OpenID Connect provider, or both at the same time. The login page shows one button per configured provider. Google works out of the box by setting HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET; for any other OIDC issuer, set HIVE_OIDC_ISSUER, HIVE_OIDC_CLIENT_ID, and optionally HIVE_OIDC_CLIENT_SECRET and HIVE_OIDC_DISPLAY_NAME. Email domains can be restricted with HIVE_GOOGLE_ALLOWED_DOMAINS and HIVE_OIDC_ALLOWED_DOMAINS.