Add equivalent of Dgraph's 'has' to GraphQL

It's currently not possible to filter in GraphQL by if a node does or does not have a particular edge.  We, should port Dgraph's `has` condition to solve that.

Expect it to look like

```graphql
queryUser(filter: { has: friends }) { 
   ... 
} 
```

or

```graphql
{
  queryUser(filter: {firstName: {eq: null}}) {
    username
    firstName
  }
}
```