Restore backwards compatibility when using older BNs (#3410)

## Issue Addressed

https://github.com/status-im/nimbus-eth2/issues/3930

## Proposed Changes

We can trivially support beacon nodes which do not provide the `is_optimistic` field by wrapping the field in an `Option`.

beacon_node/http_api/tests/tests.rs

@@ -1300,7 +1300,7 @@ impl ApiTester {
 
         let expected = SyncingData {
             is_syncing: false,
-            is_optimistic: false,
+            is_optimistic: Some(false),
             head_slot,
             sync_distance,
         };