Support WhereParser on Vertex.

Currently, WhereParser is only available for Edge so it is not possible to filter vertex with predicates.

One example query would look like followings.

query {
  s2graph {
    User(id: 1, filter: "name contains 'steamshon'") {
      id
    }
  }
}

I suggest to add vertex support in WhereParser, so GraphQL query can also benefit from WhereParser.