Multi-provider OIDC authentication with Google and generic OIDC

Hive can now delegate sign-in to one or more identity providers at the same time. Google is built in with HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET, and any generic OpenID Connect provider can be added via HIVE_OIDC_ISSUER plus HIVE_OIDC_CLIENT_ID and HIVE_OIDC_CLIENT_SECRET. Each provider appears as its own button on the login page, and email domains can be allowlisted to restrict who can sign in.