Add is_optimistic to eth/v1/node/syncing response (#3374)

## Issue Addressed

As specified in the [Beacon Chain API specs](https://github.com/ethereum/beacon-APIs/blob/master/apis/node/syncing.yaml#L32-L35) we should return `is_optimistic` as part of the response to a query for the `eth/v1/node/syncing` endpoint.

## Proposed Changes

Compute the optimistic status of the head and add it to the `SyncingData` response.

common/eth2/src/types.rs

@@ -552,6 +552,7 @@ pub struct VersionData {
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct SyncingData {
     pub is_syncing: bool,
+    pub is_optimistic: bool,
     pub head_slot: Slot,
     pub sync_distance: Slot,
 }