OIDC login support for Google and generic providers

Self-hosted instances can now authenticate users through one or more identity providers. Google is supported as a first-class provider by setting HIVE_GOOGLE_CLIENT_ID and HIVE_GOOGLE_CLIENT_SECRET, and any generic OIDC provider can be wired in with HIVE_OIDC_ISSUER, HIVE_OIDC_CLIENT_ID, and HIVE_OIDC_CLIENT_SECRET. The login screen shows a button for each configured provider, and the generic chart makes it easy to plug in your own secrets.