Add tenant config option to allow timeline_offloading (#9598)

Allow us to enable timeline offloading for single tenants without having
to enable it for the entire pageserver.

Part of #8088.

pageserver/src/tenant.rs

@@ -2499,8 +2499,15 @@ impl Tenant {
                             .iter()
                             .any(|(_id, tl)| tl.get_ancestor_timeline_id() == Some(*timeline_id))
                     };
+                    let config_allows_offload = self.conf.timeline_offloading
+                        || self
+                            .tenant_conf
+                            .load()
+                            .tenant_conf
+                            .timeline_offloading
+                            .unwrap_or_default();
                     let can_offload =
-                        can_offload && has_no_unoffloaded_children && self.conf.timeline_offloading;
+                        can_offload && has_no_unoffloaded_children && config_allows_offload;
                     if (is_active, can_offload) == (false, false) {
                         None
                     } else {
@@ -4902,6 +4909,7 @@ pub(crate) mod harness {
                 ),
                 lsn_lease_length: Some(tenant_conf.lsn_lease_length),
                 lsn_lease_length_for_ts: Some(tenant_conf.lsn_lease_length_for_ts),
+                timeline_offloading: Some(tenant_conf.timeline_offloading),
             }
         }
     }

pageserver/src/tenant/config.rs

@@ -349,6 +349,10 @@ pub struct TenantConfOpt {
     #[serde(with = "humantime_serde")]
     #[serde(default)]
     pub lsn_lease_length_for_ts: Option<Duration>,
+
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(default)]
+    pub timeline_offloading: Option<bool>,
 }
 
 impl TenantConfOpt {
@@ -411,6 +415,9 @@ impl TenantConfOpt {
             lsn_lease_length_for_ts: self
                 .lsn_lease_length_for_ts
                 .unwrap_or(global_conf.lsn_lease_length_for_ts),
+            timeline_offloading: self
+                .lazy_slru_download
+                .unwrap_or(global_conf.timeline_offloading),
         }
     }
 }
@@ -464,6 +471,7 @@ impl From<TenantConfOpt> for models::TenantConfig {
             image_layer_creation_check_threshold: value.image_layer_creation_check_threshold,
             lsn_lease_length: value.lsn_lease_length.map(humantime),
             lsn_lease_length_for_ts: value.lsn_lease_length_for_ts.map(humantime),
+            timeline_offloading: value.timeline_offloading,
         }
     }
 }