test: fix tenant duplication utility generation numbers (#8096)

## Problem
We have this set of test utilities which duplicate a tenant by copying
everything that's in remote storage and then attaching a tenant to the
pageserver and storage controller. When the "copied tenants" are created
on the storage controller, they start off from generation number 0. This
means that they can't see anything past that generation.

This issues has existed ever since generation numbers have been
introduced, but we've largely been lucky
for the generation to stay stable during the template tenant creation.

## Summary of Changes
Extend the storage controller debug attach hook to accept a generation
override. Use that in the tenant duplication logic to set the generation
number to something greater than the naturally reached generation. This
allows the tenants to see all layer files.

control_plane/src/storage_controller.rs

@@ -46,6 +46,7 @@ const STORAGE_CONTROLLER_POSTGRES_VERSION: u32 = 16;
 pub struct AttachHookRequest {
     pub tenant_shard_id: TenantShardId,
     pub node_id: Option<NodeId>,
+    pub generation_override: Option<i32>,
 }
 
 #[derive(Serialize, Deserialize)]
@@ -440,6 +441,7 @@ impl StorageController {
         let request = AttachHookRequest {
             tenant_shard_id,
             node_id: Some(pageserver_id),
+            generation_override: None,
         };
 
         let response = self