Add user state observer and ID getters

[jennantilla]

Description

One Line Summary

Add getters for onesignal ID and external ID, and a user state observer to know when these values are changed.

Details

Motivation

From developer feedback and to support integration partners, we are exposing the onesignal ID and external ID with getters.

We also want to grant developers the ability to add observers that can be called when there is a change in user state.

Testing

Manual testing

Manually tested on:

• Pixel 7 Pro Emulator running Android 14
• iPhone 15 Pro running iOS 17.2

Tested adding observer for user state and calling new methods to access externalId and onesignalId. Ensured congruency between iOS and Android responses.

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
  • 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
• 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.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• 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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