delete timelines from s3 (#4384)

Delete data from s3 when timeline deletion is requested

## Summary of changes

UploadQueue is altered to support scheduling of delete operations in
stopped state. This looks weird, and I'm thinking whether there are
better options/refactorings for upload client to make it look better.

Probably can be part of https://github.com/neondatabase/neon/issues/4378

Deletion is implemented directly in existing endpoint because changes are not
that significant. If we want more safety we can separate those or create
feature flag for new behavior.

resolves [#4193](https://github.com/neondatabase/neon/issues/4193)

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

pageserver/src/task_mgr.rs

@@ -257,6 +257,9 @@ pub enum TaskKind {
     // task that handles attaching a tenant
     Attach,
 
+    // Used mostly for background deletion from s3
+    TimelineDeletionWorker,
+
     // task that handhes metrics collection
     MetricsCollection,