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

libs/utils/src/bin_ser.rs

@@ -286,6 +286,11 @@ mod tests {
     const SHORT2_ENC_LE: &[u8] = &[8, 0, 0, 3, 7];
     const SHORT2_ENC_LE_TRAILING: &[u8] = &[8, 0, 0, 3, 7, 0xff, 0xff, 0xff];
 
+    #[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
+    struct NewTypeStruct(u32);
+    const NT1: NewTypeStruct = NewTypeStruct(414243);
+    const NT1_INNER: u32 = 414243;
+
     #[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
     pub struct LongMsg {
         pub tag: u8,
@@ -408,4 +413,42 @@ mod tests {
         let msg2 = LongMsg::des(&encoded).unwrap();
         assert_eq!(msg, msg2);
     }
+
+    #[test]
+    /// Ensure that newtype wrappers around u32 don't change the serialization format
+    fn be_nt() {
+        use super::BeSer;
+
+        assert_eq!(NT1.serialized_size().unwrap(), 4);
+
+        let msg = NT1;
+
+        let encoded = msg.ser().unwrap();
+        let expected = hex_literal::hex!("0006 5223");
+        assert_eq!(encoded, expected);
+
+        assert_eq!(encoded, NT1_INNER.ser().unwrap());
+
+        let msg2 = NewTypeStruct::des(&encoded).unwrap();
+        assert_eq!(msg, msg2);
+    }
+
+    #[test]
+    /// Ensure that newtype wrappers around u32 don't change the serialization format
+    fn le_nt() {
+        use super::LeSer;
+
+        assert_eq!(NT1.serialized_size().unwrap(), 4);
+
+        let msg = NT1;
+
+        let encoded = msg.ser().unwrap();
+        let expected = hex_literal::hex!("2352 0600");
+        assert_eq!(encoded, expected);
+
+        assert_eq!(encoded, NT1_INNER.ser().unwrap());
+
+        let msg2 = NewTypeStruct::des(&encoded).unwrap();
+        assert_eq!(msg, msg2);
+    }
 }