storcon: add force_upsert flag to timeline_import endpoint (#12622)

It is useful to have ability to update an existing timeline entry, as a
way to mirror legacy migrations to the storcon managed table.

libs/pageserver_api/src/controller_api.rs

@@ -596,6 +596,7 @@ pub struct TimelineImportRequest {
     pub timeline_id: TimelineId,
     pub start_lsn: Lsn,
     pub sk_set: Vec<NodeId>,
+    pub force_upsert: bool,
 }
 
 #[derive(serde::Serialize, serde::Deserialize, Clone)]