Extend buckets list with static list from settings

Ceph does not support ListBuckets on non-owned buckets for normal users. As our data buckets from clients are created by high-privilege accounts, we cannot assign them to a specific ceph account per instance.

But they remain usable directly, as long as we know their name.

Keep the current logic to read the user owned bucket, but also support a list coming from settings to extend it.

The setting should be ingest.extra_buckets, and default to an empty list.