storcon: Fix migration for Attached(0) tenants (#12256)

## Problem

`Attached(0)` tenant migrations can get stuck if the heatmap file has
not been uploaded.

## Summary of Changes

- Added a test to reproduce the issue.
- Introduced a `kick_secondary_downloads` config flag:
  - Enabled in testing environments.
  - Disabled in production (and in the new test).
- Updated `Attached(0)` locations to consider the number of secondaries
in their intent when deciding whether to download the heatmap.

control_plane/src/local_env.rs

@@ -211,6 +211,8 @@ pub struct NeonStorageControllerConf {
     pub use_local_compute_notifications: bool,
 
     pub timeline_safekeeper_count: Option<i64>,
+
+    pub kick_secondary_downloads: Option<bool>,
 }
 
 impl NeonStorageControllerConf {
@@ -242,6 +244,7 @@ impl Default for NeonStorageControllerConf {
             use_https_safekeeper_api: false,
             use_local_compute_notifications: true,
             timeline_safekeeper_count: None,
+            kick_secondary_downloads: None,
         }
     }
 }

control_plane/src/storage_controller.rs

@@ -557,6 +557,10 @@ impl StorageController {
             args.push("--use-local-compute-notifications".to_string());
         }
 
+        if let Some(value) = self.config.kick_secondary_downloads {
+            args.push(format!("--kick-secondary-downloads={value}"));
+        }
+
         if let Some(ssl_ca_file) = self.env.ssl_ca_cert_path() {
             args.push(format!("--ssl-ca-file={}", ssl_ca_file.to_str().unwrap()));
         }