Multiple OIDC providers including Google

Hive now supports multiple OIDC providers at the same time, with Google built in as a first-class option. Operators can configure Google via HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET, optionally restrict access to specific email domains with HIVE_GOOGLE_ALLOWED_DOMAINS, and any other OIDC provider by supplying its issuer through HIVE_OIDC_ISSUER. Each provider appears as its own button on the login screen, making it easier for teams to use their existing identity provider.