Sign in with Google or a generic OIDC provider

Hive now supports one OIDC provider per instance. Choose the Google preset using HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET, or point a generic provider at its issuer with HIVE_OIDC_ISSUER plus HIVE_OIDC_CLIENT_ID. Operators can restrict sign-in to specific email domains with HIVE_GOOGLE_ALLOWED_DOMAINS or HIVE_OIDC_ALLOWED_DOMAINS. The flow is handled by Ueberauth and ueberauth_oidcc, which discovers endpoints automatically.