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.

beacon_node/http_api/tests/tests.rs

@@ -1281,6 +1281,7 @@ impl ApiTester {
 
         let expected = SyncingData {
             is_syncing: false,
+            is_optimistic: false,
             head_slot,
             sync_distance,
         };