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

libs/pageserver_api/src/controller_api.rs

@@ -179,7 +179,6 @@ pub struct TenantDescribeResponseShard {
 /// specifies some constraints, e.g. asking it to get off particular node(s)
 #[derive(Serialize, Deserialize, Debug)]
 pub struct TenantShardMigrateRequest {
-    pub tenant_shard_id: TenantShardId,
     pub node_id: NodeId,
 }