pageserver: gate previous heatmap behind config flag (#11088)

## Problem

On unarchival, we update the previous heatmap with all visible layers.
When the primary generates a new heatmap it includes all those layers,
so the secondary will download them. Since they're not actually resident
on the primary (we didn't call the warm up API), they'll never be
evicted, so they remain in the heatmap.

This leads to oversized secondary locations like we saw in pre-prod.

## Summary of changes

Gate the loading of the previous heatmaps and the heatmap generation on
unarchival behind configuration
flags. They are disabled by default, but enabled in tests.

libs/pageserver_api/src/config.rs

@@ -123,6 +123,10 @@ pub struct ConfigToml {
     pub enable_read_path_debugging: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub validate_wal_contiguity: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub load_previous_heatmap: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub generate_unarchival_heatmap: Option<bool>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -523,6 +527,8 @@ impl Default for ConfigToml {
                 None
             },
             validate_wal_contiguity: None,
+            load_previous_heatmap: None,
+            generate_unarchival_heatmap: None,
         }
     }
 }