feat: implement as option for populate to assign results to an alternate path and add tests.

[SMK-07]

Added support for an as option in populate(). This allows you to populate a path but assign the result to a different property, keeping the original path as-is. This is useful when you want to keep the foreign key (ObjectId) but also have the populated document available on the same object, without defining duplicate fields in the schema.

Examples

const schema = new Schema({
  authorId: { type: Schema.Types.ObjectId, ref: 'User' }
});
const BlogPost = mongoose.model('BlogPost', schema);

const post = await BlogPost.findOne().populate({
  path: 'authorId',
  as: 'author'
});

// post.authorId is still the ObjectId
// post.author is the populated User document
console.log(post.authorId); // 507f1f77bcf86cd799439011
console.log(post.author.name); // 'Val'

This works with lean(), arrays, and nested paths as well.

[AbdelrahmanHafez]

Please revert all unrelated formatting changes, this is difficult to review.