feat(admin): expose KMS backend capabilities in status response

Surface the backend capability matrix as an optional, additive
'capabilities' field on the /v3/kms/status response so management
clients can discover supported lifecycle operations. The field is
skipped when unset, keeping the response shape unchanged for existing
consumers, and legacy payloads without it still deserialize.

Refs rustfs/backlog#1571 (part of rustfs/backlog#1562)

crates/kms/src/manager.rs

@@ -159,6 +159,11 @@ impl KmsManager {
     pub async fn health_check(&self) -> Result<bool> {
         self.backend.health_check().await
     }
+
+    /// Report the capabilities of the configured backend
+    pub fn backend_capabilities(&self) -> crate::backends::BackendCapabilities {
+        self.backend.capabilities()
+    }
 }
 
 #[cfg(test)]