Give the ability to have an array for foreignField

**Do you want to request a *feature* or report a *bug*?**
feature

**What is the current behavior?**
When you pass a String to localField and an array to foreignField property in Schema, the population will failed

**What is the expected behavior?**
I would like to be able to set an array of field name for foreignField when the localField is a String. Previously an issue has been created to request a feature to have an array for localField and foreignField, I know... But the meaning to have a String for localField and an array for foreignField isn't the same...
When we have a String for localField and an array for foreignField that would meaning that I want all document wich have, at least, one of the property in the foreignField array equals to the localField value.

Currently, to support this feature only two lines ( one is the typing for typescript ) has to be changed :
- https://github.com/Automattic/mongoose/blob/master/lib/helpers/populate/getModelsMapForPopulate.js#L548
We just have to replace this line by :
```
foreignField: new Set(Array.isArray(data.foreignField) ? data.foreignField : [data.foreignField])
```

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
Node.js: 16.11.0
Mongoose: 6.2.0

Start in lib/helpers/populate/getModelsMapForPopulate.js around line 548, and read the surrounding populate mapping logic and the TypeScript declaration mentioned in the issue. Confirm how a string localField should match any field in an array foreignField, then verify that the existing populate behavior remains unchanged for string foreignField values.

Written by the indexing model from the issue text.