fix(GraphQL): optimize eq filter queries

[minhaj-shakeel]

Fixes GQLSAAS-1236.
This PR optimizes eq filter GraphQL queries.
For ex:
The below DQL query:

query {
  queryPerson(filter: {nick: {eq: "Dgraph"}}){
    id
    name
    nick
  }
}

will now be written into:

query {
  queryPerson(func: eq(Person.nick, "Dgraph")) @filter(type(Person)) {
    Person.id : uid
    Person.name : Person.name
    Person.nick : Person.nick
  }
}

which was earlier written into:-

query {
  queryPerson(func: type(Person)) @filter(eq(Person.nick, "Dgraph")){
    Person.id : uid
    Person.name : Person.name
    Person.nick : Person.nick
  }
}

---

This change is 

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @minhaj-shakeel)

[matthewmcneely]

We should point out in the release notes that the optimization is not realized when complex filters (AND, NOT, OR) are present in the GraphQL query. For instance see the "Filter with nested 'and'" test in query_test.yaml