federation gateway should throw on type's conflicts #826

Given the following schemas:

Node 1

    extend type Query {
      getOrder: Order
    }

    type Order @key(fields: "id") {
      id: ID!
      name: String
    }

Node 2

  type Order @key(fields: "id") {
    id: ID!
    surname: String
  }

The gateway starts correctly instead of throwing.

This is a clear developer's error, as the Node 2's schema should use the extends keyword:

  extends type Order {
    surname: String
  }

In this case, Mercurius should throw an error when:

• there is a naming conflict without the extends keyword
• the extends entity must not use the ID! property value

In both the above cases, Apollo server trigger this error:

This data graph is missing a valid configuration. Field "Order.id" can only be defined once.

There can be only one type named "Order".

This would help developers to spot these schema conflicts at first gateway run.