add directives an model via annotations

[heiko-he]

It should exist a way to add some/own directives to the model with annotations.

E.g:

@GraphQlFederationKey(fields = "id")
@GraphQlFederationExtends
@GraphQLType(name = "Product")
public class ExtensionProduct {
@GraphQlFederationExternal
long id;

--> the result is after generation:

extend type Product @key(fields: "id"){
id: ID! @external
}

Thats some directives they will be used in federational schema - Apollo Federation:
https://www.apollographql.com/docs/apollo-server/federation/introduction/