Add timeline offload mechanism (#8907)

Implements an initial mechanism for offloading of archived timelines.

Offloading is implemented as specified in the RFC.

For now, there is no persistence, so a restart of the pageserver will
retrigger downloads until the timeline is offloaded again.

We trigger offloading in the compaction loop because we need the signal
for whether compaction is done and everything has been uploaded or not.

Part of #8088

pageserver/src/http/routes.rs

@@ -703,6 +703,8 @@ async fn timeline_archival_config_handler(
     let tenant_shard_id: TenantShardId = parse_request_param(&request, "tenant_shard_id")?;
     let timeline_id: TimelineId = parse_request_param(&request, "timeline_id")?;
 
+    let ctx = RequestContext::new(TaskKind::MgmtRequest, DownloadBehavior::Warn);
+
     let request_data: TimelineArchivalConfigRequest = json_request(&mut request).await?;
     check_permission(&request, Some(tenant_shard_id.tenant_id))?;
     let state = get_state(&request);
@@ -713,7 +715,7 @@ async fn timeline_archival_config_handler(
             .get_attached_tenant_shard(tenant_shard_id)?;
 
         tenant
-            .apply_timeline_archival_config(timeline_id, request_data.state)
+            .apply_timeline_archival_config(timeline_id, request_data.state, ctx)
             .await?;
         Ok::<_, ApiError>(())
     }