remote_storage: expose last_modified in listings (#8497)

## Problem

The scrubber would like to check the highest mtime in a tenant's objects
as a safety check during purges. It recently switched to use
GenericRemoteStorage, so we need to expose that in the listing methods.

## Summary of changes

- In Listing.keys, return a ListingObject{} including a last_modified
field, instead of a RemotePath

---------

Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>

pageserver/src/tenant/mgr.rs

@@ -1407,6 +1407,7 @@ impl TenantManager {
             tracing::info!("Remote storage already deleted");
         } else {
             tracing::info!("Deleting {} keys from remote storage", keys.len());
+            let keys = keys.into_iter().map(|o| o.key).collect::<Vec<_>>();
             self.resources
                 .remote_storage
                 .delete_objects(&keys, &self.cancel)

pageserver/src/tenant/remote_timeline_client.rs

@@ -1380,12 +1380,13 @@ impl RemoteTimelineClient {
         // marker via its deleted_at attribute
         let latest_index = remaining
             .iter()
-            .filter(|p| {
-                p.object_name()
+            .filter(|o| {
+                o.key
+                    .object_name()
                     .map(|n| n.starts_with(IndexPart::FILE_NAME))
                     .unwrap_or(false)
             })
-            .filter_map(|path| parse_remote_index_path(path.clone()).map(|gen| (path, gen)))
+            .filter_map(|o| parse_remote_index_path(o.key.clone()).map(|gen| (o.key.clone(), gen)))
             .max_by_key(|i| i.1)
             .map(|i| i.0.clone())
             .unwrap_or(
@@ -1396,14 +1397,12 @@ impl RemoteTimelineClient {
 
         let remaining_layers: Vec<RemotePath> = remaining
             .into_iter()
-            .filter(|p| {
-                if p == &latest_index {
-                    return false;
+            .filter_map(|o| {
+                if o.key == latest_index || o.key.object_name() == Some(INITDB_PRESERVED_PATH) {
+                    None
+                } else {
+                    Some(o.key)
                 }
-                if p.object_name() == Some(INITDB_PRESERVED_PATH) {
-                    return false;
-                }
-                true
             })
             .inspect(|path| {
                 if let Some(name) = path.object_name() {

pageserver/src/tenant/remote_timeline_client/download.rs

@@ -295,10 +295,11 @@ where
         };
     }
 
-    for key in listing.keys {
-        let object_name = key
+    for object in listing.keys {
+        let object_name = object
+            .key
             .object_name()
-            .ok_or_else(|| anyhow::anyhow!("object name for key {key}"))?;
+            .ok_or_else(|| anyhow::anyhow!("object name for key {}", object.key))?;
         other_prefixes.insert(object_name.to_string());
     }
 
@@ -459,7 +460,7 @@ pub(crate) async fn download_index_part(
     // is <= our own.  See "Finding the remote indices for timelines" in docs/rfcs/025-generation-numbers.md
     let max_previous_generation = indices
         .into_iter()
-        .filter_map(parse_remote_index_path)
+        .filter_map(|o| parse_remote_index_path(o.key))
         .filter(|g| g <= &my_generation)
         .max();