Add endpoint and storcon cli cmd to set sk scheduling policy (#10400)

Implementing the last missing endpoint of #9981, this adds support to
set the scheduling policy of an individual safekeeper, as specified in
the RFC. However, unlike in the RFC we call the endpoint
`scheduling_policy` not `status`

Closes #9981.

As for why not use the upsert endpoint for this: we want to have the
safekeeper upsert endpoint be used for testing and for deploying new
safekeepers, but not for changes of the scheduling policy. We don't want
to change any of the other fields when marking a safekeeper as
decommissioned for example, so we'd have to first fetch them only to
then specify them again. Of course one can also design an endpoint where
one can omit any field and it doesn't get modified, but it's still not
great for observability to put everything into one big "change something
about this safekeeper" endpoint.

storage_controller/src/persistence.rs

@@ -1104,6 +1104,37 @@ impl Persistence {
         })
         .await
     }
+
+    pub(crate) async fn set_safekeeper_scheduling_policy(
+        &self,
+        id_: i64,
+        scheduling_policy_: SkSchedulingPolicy,
+    ) -> Result<(), DatabaseError> {
+        use crate::schema::safekeepers::dsl::*;
+
+        self.with_conn(move |conn| -> DatabaseResult<()> {
+            #[derive(Insertable, AsChangeset)]
+            #[diesel(table_name = crate::schema::safekeepers)]
+            struct UpdateSkSchedulingPolicy<'a> {
+                id: i64,
+                scheduling_policy: &'a str,
+            }
+            let scheduling_policy_ = String::from(scheduling_policy_);
+
+            let rows_affected = diesel::update(safekeepers.filter(id.eq(id_)))
+                .set(scheduling_policy.eq(scheduling_policy_))
+                .execute(conn)?;
+
+            if rows_affected != 1 {
+                return Err(DatabaseError::Logical(format!(
+                    "unexpected number of rows ({rows_affected})",
+                )));
+            }
+
+            Ok(())
+        })
+        .await
+    }
 }
 
 /// Parts of [`crate::tenant_shard::TenantShard`] that are stored durably