add IsBeaconSynced to node status

[lrettig]

> grpcurl -plaintext localhost:9092 spacemesh.v1.NodeService.Status
{
  "status": {
    "connectedPeers": "1",
    "isSynced": true,
    "syncedLayer": {
      "number": 19386
    },
    "topLayer": {
      "number": 19389
    },
    "verifiedLayer": {
      "number": 19383
    }
  }
}

This currently includes isSynced but not IsBeaconSynced which can also block hare from running:

https://github.com/spacemeshos/go-spacemesh/blob/27c3dd7c2e2093bba9721fee789d54122783573c/hare/broker.go#L425-L428