Timeline offloading persistence (#9444)

Persist timeline offloaded state to S3.

Right now, as of #8907, at each restart of the pageserver, all offloaded
state is lost, so we load the full timeline again. As it starts with an
empty local directory, we might potentially download some files again,
leading to downloads that are ultimately wasteful.

This patch adds support for persisting the offloaded state, allowing us
to never load offloaded timelines in the first place. The persistence
feature is facilitated via a new file in S3 that is tenant-global, which
contains a list of all offloaded timelines. It is updated each time we
offload or unoffload a timeline, and otherwise never touched.

This choice means that tenants where no offloading is happening will not
immediately get a manifest, keeping the change very minimal at the
start.

We leave generation support for future work. It is important to support
generations, as in the worst case, the manifest might be overwritten by
an older generation after a timeline has been unoffloaded (and
unarchived), so the next pageserver process instantiation might wrongly
believe that some timeline is still offloaded even though it should be
active.

Part of #9386, #8088

libs/pageserver_api/src/models.rs

@@ -699,6 +699,8 @@ pub struct OffloadedTimelineInfo {
     pub ancestor_timeline_id: Option<TimelineId>,
     /// Whether to retain the branch lsn at the ancestor or not
     pub ancestor_retain_lsn: Option<Lsn>,
+    /// The time point when the timeline was archived
+    pub archived_at: chrono::DateTime<chrono::Utc>,
 }
 
 /// This represents the output of the "timeline_detail" and "timeline_list" API calls.