Minimal support for external SSO-based keys

I know the history of #6949 and the security concerns around key-connector. I'd like to propose a another approach from what was in included in that PR - a minimal set of changes to VW that would allow it to work with a key-connector service implemented elsewhere. The goal being that there is only a small footprint in this codebase, locked down by default, to ensure there is minimal maintenance added to @dani-garcia and other maintainers. I think the changes to vaultwarden would roughly required (based on a little spike I built): config.rs • New setting sso_key_connector_url which would be the URL for the connector service • New setting for an sso JWT claim to indicate the user is required to use key-connector schema.rs • One DB column to track users.uses_key_connector new api/core/key_connector.rs • Two new API endpoints - /accounts/set-key-connector-key and /accounts/key-connector/confirmation-details/<org> to allow the web client t…