This adds an API to the storage controller to list safekeepers
registered to it.
This PR does a `diesel print-schema > storage_controller/src/schema.rs`
because of an inconsistency between up.sql and schema.rs, introduced by
[this](2c142f14f7)
commit, so there is some updates of `schema.rs` due to that. As a
followup to this, we should maybe think about running `diesel
print-schema` in CI.
Part of #9981
342cbea255