[Fix] Set External User Id on iOS

[tanaynigam]

Description

One Line Summary

Fix Set External User Id not working on iOS

Details

Motivation

Temporary fix to set external user id not working on iOS. The previous implementation uses a single SetExternalUserId method with optional nullable parameter hashToken in Xamarin. However, OneSignal iOS native SDK does not support nullable hashToken as a parameter. This fix uses temporary function overloading to allow users to continue using SetExternalUserId without a hashToken

Scope

• Added SetExternalUserId method without hashToken parameter to iOS, Android and Core projects

Issues this resolves

• Fixes OneSignal.Default.SetExternalUserId value cannot be null - Parameter name hashtoken #314

Testing

Manual testing

• MacOS 12.4
• Visual Studio 2022 Version 17.05
• .Net 6.0.0.262
• Xamarin.iOS 15.10.0.5
• Xamarin.Forms project
• Created and tested a nuget package on an iOS device

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

---

This change is 

[jkasten2]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @tanaynigam)