Support for equality and inequality in filter when a predicate doens't have index

[mangalaman93]

We already support regular expressions in filter even when a predicate doesn't have trigram index. We could further enhance and add support for more functions in filter when we don't have indexes for the predicates used in the filter.

Schema

firstName: string .
lastName: string .

Mutation

{
  set {
    _:luke <firstName> "Luke" .
    _:luke <lastName> "Skywalker" .
    _:leia <firstName> "Princess" .
    _:leia <lastName> "Leia" .
    _:han <firstName> "Han" .
    _:han <lastName> "Solo" .
    _:har <firstName> "Harrison" .
    _:har <lastName> "Ford" .
    _:ss <firstName> "Steven" .
    _:ss <lastName> "Spielberg" .
  }
}

Query

{
  response(func: has(firstName)) @filter(eq(firstName, "Han")) {
    uid
    firstName
    lastName
  }
}

Fails with following error:

{
  "name": "t",
  "url": "http://localhost:8180/query?timeout=60s",
  "errors": [
    {
      "message": ": Attribute firstName is not indexed.",
      "extensions": {
        "code": "ErrorInvalidRequest"
      }
    }
  ]
}

Related to #4065 #4030 a4756c2

[mangalaman93]

duplicate of #4305