feature request: support of GraphQL interfaces

[gauthiermyr]

Currently envio does not support the use of interfaces in the GraphQL schema (see: https://graphql.org/learn/schema/#interface-types).

Interfaces allow this type of data structure and help in indexing entities with a generic structure and contracts with inheritance

Example:

interface ITest {
  id: ID!
  common: String!
}

type TestA implements ITest {
  id: ID!
  common: String!

  a: String!
}

type TestB implements ITest {
  id: ID!
  common: String!

  b: String!
}

type Entity {
  id: ID!

  test: ITest
}

query getEntity($id: ID!) {
  entity(id: $id) {
    id
    test {
      common
      ... on TestA {
        id
        a
      }
      ... on TestB {
        id
        b
      }
    }
  }
}