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.

libs/utils/src/auth.rs

@@ -40,6 +40,11 @@ pub enum Scope {
     /// Allows access to storage controller APIs used by the scrubber, to interrogate the state
     /// of a tenant & post scrub results.
     Scrubber,
+
+    /// This scope is used for communication with other storage controller instances.
+    /// At the time of writing, this is only used for the step down request.
+    #[serde(rename = "controller_peer")]
+    ControllerPeer,
 }
 
 /// JWT payload. See docs/authentication.md for the format