Allow creating related items within single create mutation

[jesstelford]

Example

keystone.addList('User', {
  fields: {
    posts: { type: Relationship, ref: 'Post', many: true },
    org: { type: Relationship, ref: 'Org' },
  }
});

keystone.addList('Org', {
  fields: {
    name: { type: Text },
  }
});

keystone.addList('Post', {
  fields: {
    title: { type: Text },
  }
});

Current Output (after #199 is merged)

type CreateUserInput {
  posts: [ID]
}

type Mutation {
  createUser(data: CreateUserInput): User
}

Suggested Output

type CreateUserInput {
  posts: [UpdateOrCreatePostInput]
}

# NOTE: different input type as defined in https://github.com/keystonejs/keystone-5/issues/182
input LinkOrCreatePostInput {
  id: ID # NOTE: Can't pass both id AND create
  create: CreatePostInput
}

type Mutation {
  createUser(data: CreateUserInput): User
}

---

See also keystonejs/keystone-5#182 & keystonejs/keystone-5#185

[jesstelford]

The same applies to the updateX mutations.