Add getters and watchers for oneSignalId and externalId

[jinliu9508]

Description

One Line Summary

Add getters and observer for onesignalId and externalId.

Details

Motivation

Exposing the onesignal ID and external ID with getters. Also grant developers the ability to add observers that can be called when there is a change in user state.

Scope

UserState contains onesignalId and externalId (both can be empty strings if not available or not set).

Testing

Unit testing

No unit testing at this time

Manual testing

I have added two testing buttons in the test scene for getting onesignalId and externalId. I have also add observer to test the callback when the ids are changed.

Unity version: 2022.3.10f1 For Android: Emulator Pixel 3a api34 For ios: iPhone15 Simulator

Affected code checklist

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

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• [x] Any Public API changes are explained in the PR details and conform to existing APIs

Testing

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

Final pass

• [x] Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• [x] I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is 

[jinliu9508]

Just had some nits, looks good!

Could you also update the Manual testing section in your PR to include what devices and version of Unity you tested on? I think it would be helpful to have it documented

WIP: I will test the PR on devices later

@shepherd-l Thanks for the review! I have made changes addressing all the concerns. Please have a look again.