Fix GRAPHQL-905.
This PR implements [Federation Schema Specification](https://www.apollographql.com/docs/federation/federation-spec/#federation-schema-specification) in order to support apollo federation.

Fixes GRAPHQL-934.
This PR adds `_service` resolver according to the federation specs which return generated schema string without @apollo directives to be used by the gateway.
The query is:
```
query {
   _service {
      sdl
   }
}
```
which returns the schema string.

Fixes GRAPHQL-951.
This PR modifies GraphQL generated schema in order to support federation.
It also removes some directive definitions which currently give validation error in apollo.
This PR also removes generated mutations and mutation input types from the Generated schema.

Fixes GRAPHQL-922.
This PR adds `entity` resolver which resolves `_entities(representation: [Any]!)` queries which are sent by the gateway to the graphql service.

Fixes GRAPHQL-968.
This PR does two things.
1 - It exposes the `mutations` to the apollo gateway which were earlier restricted. It exposes mutations for both `type definitions` as well as `type extensions`. Whereas `queries` for the `extended types` are `not exposed` as they are resolved through `entity resolver`.
2 - It adds support for add mutation on extended type having field with `ID` type as @key field. In this case the field is stored internally in the dgraph as `string` type so its value needs to be given as an argument inside the mutation.

…haj/apollo-federation

…io/dgraph into minhaj/apollo-federation