Add removed_at to FlowTypeSetting

[Taucher2003]

Because user configuration (FlowSetting) can point to a FlowTypeSetting that is getting removed by the runtime via the sagittarius.FlowTypeService.Update grpc call, we can't delete those objects directly.

Instead they must remain stored in the database with a removed_at date, so they can be cleaned up in the background after nothing references them anymore.