Squashed commit of the following:

commit de90ba56d4
Author: Christian Schwarz <christian@neon.tech>
Date:   Mon Nov 27 14:47:26 2023 +0000

    expose generation number in API

commit ae2c7589f9
Author: Christian Schwarz <christian@neon.tech>
Date:   Mon Nov 27 14:53:13 2023 +0000

    pagectl: add subcommand to rewrite layer file history

libs/pageserver_api/src/models.rs

@@ -368,6 +368,8 @@ pub struct TenantInfo {
     /// If a layer is present in both local FS and S3, it counts only once.
     pub current_physical_size: Option<u64>, // physical size is only included in `tenant_status` endpoint
     pub attachment_status: TenantAttachmentStatus,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub generation: Option<u32>,
 }
 
 #[derive(Serialize, Deserialize, Clone)]
@@ -909,6 +911,7 @@ mod tests {
             state: TenantState::Active,
             current_physical_size: Some(42),
             attachment_status: TenantAttachmentStatus::Attached,
+            generation: None,
         };
         let expected_active = json!({
             "id": original_active.id.to_string(),
@@ -929,6 +932,7 @@ mod tests {
             },
             current_physical_size: Some(42),
             attachment_status: TenantAttachmentStatus::Attached,
+            generation: None,
         };
         let expected_broken = json!({
             "id": original_broken.id.to_string(),