types: $addFields is broken with nested fields

[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

16.6

MongoDB server version

5.x

Description

In MongoDB the following are valid expressions, but throw in our TS types.

const nestedAddFieldsExpr: PipelineStage.AddFields = {
  $addFields: {
    parent: {
      nestedField: { $add: ['$a', '$b'] }
    }
  }
};
const addFieldsArrayElemAt: PipelineStage.AddFields = {
  $addFields: {
    parent: {
      child: { $arrayElemAt: ['$someArray', 0] }
    }
  }
};

The current types only support flat-level fields, and the only way we can achieve that is by changing it in the code to:

'parentPath.nestedPath': { $add: [1, 2] }

Expected Behavior

It should support nested paths.

[vkarpov15]

Looks like this was fixed by same fix as #12096 , fix will be in 6.4.5