Don't require loki.objectStorage.endpoint for S3 config

Feature Request

Why

I would like to configure Loki for IRSA instead of using. However, Loki has a known issue that prevents setting endpoint when using IRSA: https://github.com/grafana/loki/issues/13780#issuecomment-2553760460

Proposed Solution

Remove endpoint from the required options for loki.objectStorage's schema