storcon: add peer token for peer to peer communication (#9695)

## Problem

We wish to stop using admin tokens in the infra repo, but step down
requests use the admin token.

## Summary of Changes

Introduce a new "ControllerPeer" scope and use it for step-down requests.

safekeeper/src/auth.rs

@@ -20,7 +20,8 @@ pub fn check_permission(claims: &Claims, tenant_id: Option<TenantId>) -> Result<
             | Scope::PageServerApi
             | Scope::GenerationsApi
             | Scope::Infra
-            | Scope::Scrubber,
+            | Scope::Scrubber
+            | Scope::ControllerPeer,
             _,
         ) => Err(AuthError(
             format!(