Defer: split queries into primary and deferred queries for format_response

[bnjjj]

Related to #80

The query is split regarding the @defer directive. If we could statically detect that the selection will be deferred then we just skip it from the primary selection. If not we will clone the selection into deferred_queries and also keep it in the primary query to be able to detect at the execution stage if it's a deferred selection or not. For now we split all deferred queries either if it's an entity or just a field of an entity. It might change.

For example with that schema:

type Query {
    foo: String
    stuff: Bar
    array: [Bar]
    baz: String
    other: String
}
type Bar {
    bar: String
    baz: String
    iz: Iz
}
type Iz {
    field: String
    another: String
}

with this query:

query Test {
    baz
    array {
        bar
    }
    ... @defer {
        foo
        other
        stuff {
            baz
            ... @defer {
                bar
                iz {
                    field
                    another
                }
            }
        }
    }
}

We should have 1 primary query:

query Test {
    baz
    array {
        bar
    }
}

and 2 deferred queries:

{
    foo
    other
    stuff {
        baz
    }
}

{
    bar
    iz {
        field
        another
    }
}

[bnjjj]

@pcmanus could you correct me if I'm wrong.please ?

[bnjjj]

Not useful anymore since we can fetch it from the query planner in subselection field