bug: throw validation error when batchKey is present in resolver and that field is of list type.

[laststylebender14]

in this scenario, batchKey doesn't make sense as this resolver will always return single user, so raise validation error here.

type Query {
  user(userIds: [Int]): [User]
    @http(
      url: "http://jsonplaceholder.typicode.com/users"
      query: [{key: "id", value: "{{.args.userIds}}"}]
      batchKey: ["id"]
    )
}