move pull_timeline to safekeeper_api and add SafekeeperGeneration (#10863)

Preparations for a successor of #10440: 

* move `pull_timeline` to `safekeeper_api` and add it to
`SafekeeperClient`. we want to do `pull_timeline` on any creations that
we couldn't do initially.
* Add a `SafekeeperGeneration` type instead of relying on a type alias.
we want to maintain a safekeeper specific generation number now in the
storcon database. A separate type is important to make it impossible to
mix it up with the tenant's pageserver specific generation number. We
absolutely want to avoid that for correctness reasons. If someone mixes
up a safekeeper and pageserver id (both use the `NodeId` type), that's
bad but there is no wrong generations flying around.

part of #9011

storage_controller/src/safekeeper_client.rs

@@ -1,5 +1,8 @@
 use crate::metrics::PageserverRequestLabelGroup;
-use safekeeper_api::models::{SafekeeperUtilization, TimelineCreateRequest, TimelineStatus};
+use safekeeper_api::models::{
+    PullTimelineRequest, PullTimelineResponse, SafekeeperUtilization, TimelineCreateRequest,
+    TimelineStatus,
+};
 use safekeeper_client::mgmt_api::{Client, Result};
 use utils::{
     id::{NodeId, TenantId, TimelineId},
@@ -94,6 +97,19 @@ impl SafekeeperClient {
         )
     }
 
+    #[allow(dead_code)]
+    pub(crate) async fn pull_timeline(
+        &self,
+        req: &PullTimelineRequest,
+    ) -> Result<PullTimelineResponse> {
+        measured_request!(
+            "pull_timeline",
+            crate::metrics::Method::Post,
+            &self.node_id_label,
+            self.inner.pull_timeline(req).await
+        )
+    }
+
     pub(crate) async fn get_utilization(&self) -> Result<SafekeeperUtilization> {
         measured_request!(
             "utilization",