pageserver: implement secondary-mode downloads (#6123)

Follows on from #6050 , in which we upload heatmaps. Secondary locations
will now poll those heatmaps and download layers mentioned in the
heatmap.

TODO:
- [X] ~Unify/reconcile stats for behind-schedule execution with
warn_when_period_overrun
(https://github.com/neondatabase/neon/pull/6050#discussion_r1426560695)~
- [x] Give downloads their own concurrency config independent of uploads

Deferred optimizations:
- https://github.com/neondatabase/neon/issues/6199
- https://github.com/neondatabase/neon/issues/6200

Eviction will be the next PR:
- #5342

pageserver/src/task_mgr.rs

@@ -258,6 +258,9 @@ pub enum TaskKind {
     /// See [`crate::disk_usage_eviction_task`].
     DiskUsageEviction,
 
+    /// See [`crate::tenant::secondary`].
+    SecondaryDownloads,
+
     /// See [`crate::tenant::secondary`].
     SecondaryUploads,