Typings: Allow setting string or ObjectID for ref relation in DocumentDefinition<T>

[ktutnik]

Do you want to request a feature or report a bug?
Feature request (no rush)

What is the current behavior?
Setting relation value with string or ObjectID not allowed in all methods using DocumentDefinition<T> such as Model.create, Model.findByIdAndUpdate.

If the current behavior is a bug, please provide the steps to reproduce.

// parent schema
Schema({
  _id: Schema.Types.ObjectId,
  name: String,
  child: { type: Schema.Types.ObjectId, ref: 'Child' }
});

// setting child with string causing compile error
await ParentModel.create({ name: "John", child: MONGODB_ID_STRING })

What is the expected behavior?
Should allowed since its allowed in JS version

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Mongoose 5.11.5