storage controller: API + CLI for migrating secondary locations (#10284)

## Problem

Currently, if we want to move a secondary there isn't a neat way to do
that: we just have migration API for the attached location, and it is
only clean to use that if you've manually created a secondary via
pageserver API in the place you're going to move it to.

Secondary migration API enables:
- Moving the secondary somewhere because we would like to later move the
attached location there.
- Move the secondary location because we just want to reclaim some disk
space from its current location.

## Summary of changes

- Add `/migrate_secondary` API
- Add `tenant-shard-migrate-secondary` CLI
- Add tests for above

control_plane/storcon_cli/src/main.rs

@@ -112,6 +112,13 @@ enum Command {
         #[arg(long)]
         node: NodeId,
     },
+    /// Migrate the secondary location for a tenant shard to a specific pageserver.
+    TenantShardMigrateSecondary {
+        #[arg(long)]
+        tenant_shard_id: TenantShardId,
+        #[arg(long)]
+        node: NodeId,
+    },
     /// Cancel any ongoing reconciliation for this shard
     TenantShardCancelReconcile {
         #[arg(long)]
@@ -540,10 +547,7 @@ async fn main() -> anyhow::Result<()> {
             tenant_shard_id,
             node,
         } => {
-            let req = TenantShardMigrateRequest {
-                tenant_shard_id,
-                node_id: node,
-            };
+            let req = TenantShardMigrateRequest { node_id: node };
 
             storcon_client
                 .dispatch::<TenantShardMigrateRequest, TenantShardMigrateResponse>(
@@ -553,6 +557,20 @@ async fn main() -> anyhow::Result<()> {
                 )
                 .await?;
         }
+        Command::TenantShardMigrateSecondary {
+            tenant_shard_id,
+            node,
+        } => {
+            let req = TenantShardMigrateRequest { node_id: node };
+
+            storcon_client
+                .dispatch::<TenantShardMigrateRequest, TenantShardMigrateResponse>(
+                    Method::PUT,
+                    format!("control/v1/tenant/{tenant_shard_id}/migrate_secondary"),
+                    Some(req),
+                )
+                .await?;
+        }
         Command::TenantShardCancelReconcile { tenant_shard_id } => {
             storcon_client
                 .dispatch::<(), ()>(
@@ -915,10 +933,7 @@ async fn main() -> anyhow::Result<()> {
                             .dispatch::<TenantShardMigrateRequest, TenantShardMigrateResponse>(
                                 Method::PUT,
                                 format!("control/v1/tenant/{}/migrate", mv.tenant_shard_id),
-                                Some(TenantShardMigrateRequest {
-                                    tenant_shard_id: mv.tenant_shard_id,
-                                    node_id: mv.to,
-                                }),
+                                Some(TenantShardMigrateRequest { node_id: mv.to }),
                             )
                             .await
                             .map_err(|e| (mv.tenant_shard_id, mv.from, mv.to, e))