tests: add infra and test for storcon leadership transfer (#8587)

## Problem
https://github.com/neondatabase/neon/pull/8588 implemented the mechanism
for storage controller
leadership transfers. However, there's no tests that exercise the
behaviour.

## Summary of changes
1. Teach `neon_local` how to handle multiple storage controller
instances. Each storage controller
instance gets its own subdirectory (`storage_controller_1, ...`).
`storage_controller start|stop` subcommands
have also been extended to optionally accept an instance id.
2. Add a storage controller proxy test fixture. It's a basic HTTP server
that forwards requests from pageserver
and test env to the currently configured storage controller.
3. Add a test which exercises storage controller leadership transfer.
4. Finally fix a couple bugs that the test surfaced

storage_controller/src/peer_client.rs

@@ -1,7 +1,7 @@
 use crate::tenant_shard::ObservedState;
 use pageserver_api::shard::TenantShardId;
 use serde::{Deserialize, Serialize};
-use std::collections::HashMap;
+use std::{collections::HashMap, time::Duration};
 use tokio_util::sync::CancellationToken;
 
 use hyper::Uri;
@@ -69,6 +69,8 @@ impl PeerClient {
             req
         };
 
+        let req = req.timeout(Duration::from_secs(2));
+
         let res = req
             .send()
             .await