Add OpenStack Swift support to blocks storage

[pracucci]

Thanos supports OpenStack Swift so we could easily support it too for the blocks storage, but currently lack some config mapping and bucket client initialisation code. We should do it.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid and help wanted! Should be pretty easy to do.

[pracucci]

OpenStack Swift is already supported in Thanos object store client (which we use in the Cortex blocks storage), so adding it would just be a config mapping, like we did for S3, GCS and Azure.