types($addToSet) is readonly

[AbdelrahmanHafez]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.4

Node.js version

16.x

MongoDB server version

5.x

Description

Currently, $addToSet is readonly which disallows people from constructing the $addToSet programatically.

Steps to Reproduce

function addToSetMutable() {
  interface IUser{
    friendsNames: string[];
  }
  const userSchema = new Schema<IUser>({
    friendsNames: [String]
  });

  const update: UpdateQuery<IUser> = { $addToSet: { friendsNames: 'John Doe' } };
  update.$addToSet.friendsNames = 'Jane Doe';
}

Expected Behavior

It should be mutable, to be able to programatically construct the $addToSet object.
One challenge is that we're using $addToSet from mongodb types, so changing that can be a bit tricky.