Feature Request: AppEvents supports external_id tracking

[tanapoln]

Checklist

• I've updated to the latest released version of the SDK
• I've searched for existing feature requests on GitHub issues
• I've read the Code of Conduct
• I've given my issue the title: Feature Request: [name of my feature request]

Goals

What do you want to achieve?
Conversion API for Web with Advanced Matching can set external_id to match the user with custom id. Currently, Facebook SDK for iOS is not supported for external_id field.

See the list of user data in the table here: https://developers.facebook.com/docs/app-events/advanced-matching#ios

Expected Results

What do you expect to happen?

Facebook SDK for iOS should support external_id field

[rromanchuk]

@tanapoln ok so i'm not going crazy. I noticed this from events manager diagnostics tab for server to server conversions via marketing api.

I always assumed that this accessor https://github.com/facebook/facebook-ios-sdk/blob/master/FBSDKCoreKit/FBSDKCoreKit/AppEvents/FBSDKAppEvents.h#L454 was responsible for setting external_id in the payload but it looks like this indeed not the case.

AppEvents.userID = "my-user-id

[Oliverccccct]

We started to support it from v9.3.0.