Feature: add support for interface resolution

[MatteoGranziera]

This implementation will enable the resolution of a GraphQL interface requested by the federation.

The resolution will be accessible using the entity resolution by putting as typename the interface as the following example

Node schema

type Query {
  product: Product
}

interface Product @key(fields: "id") {
  id: ID!
  name: String!
}

type BundleProduct implements Product @key(fields: "id") {
  id: ID!
  name: String!
  productIds: [ID!]!
}

Query

query {
    _entities(representations: [{ __typename: "Product", id: "1"}]) {
      __typename
      ... on Product {
        id
        name
      }
      ... on BundleProduct {
        productIds
      }
    }
  }

Note
The current implementation of mercurius does not allow to define an interface without its implementations but it could be a future improvement. Anyway it's possible to use this feature by import the @interfaceObject directive from Apollo Federation v2.3

[mcollina]

lgtm

[marco-ippolito]

Suggested change

	const resolveReference = resolvedType.resolveReference
	const resolveReference = resolvedType.resolveReference || function defaultResolveReference () {
	return reference
	}

[MatteoGranziera]

Done

[marco-ippolito]

I dont understand this:
first we check isInterfaceType(type) and we throw
then we check !isInterfaceType(type) and we return

[MatteoGranziera]

The first check verifies that the type is allowed to continue the type resolution (should be an object or an interface). The second if discriminate between the object and the interface, when is an object type there is no need to call resolveType function because the typename is already defined.

[marco-ippolito]

we can shorthand this

  const resolveTypeFn = type.resolveType
    ? type.resolveType
    : defaultTypeResolver

with

 const resolveTypeFn = type.resolveType || defaultTypeResolver;

[MatteoGranziera]

Done

[codeflyer]

LGTM

[marco-ippolito]

LGTM (@mcollina I dont have permissions on this repo for some reason)

[MatteoGranziera]

I've removed by accident jonnygreen as reviewer
@mcollina

[jonnydgreen]

Apologies for the delay in reviewing, LGTM!