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`.

common/eth2/src/types.rs

@@ -552,7 +552,7 @@ pub struct VersionData {
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct SyncingData {
     pub is_syncing: bool,
-    pub is_optimistic: bool,
+    pub is_optimistic: Option<bool>,
     pub head_slot: Slot,
     pub sync_distance: Slot,
 }