Add OneSignalId and ExternalId getters as well as a user changed observer

[emawby]

Description

One Line Summary

Add OneSignalId and ExternalId getters as well as a user changed observer.

Details

Add User.onesignalId and User.externalId

Also adds User.changed observer to handle changes to either id.

Motivation

Allow apps to get the current OneSignalIdd and external id. Since they can be null we also are adding the ability to observe when the OneSignalId and external id change (including null to non-null).

Scope

New methods on the user namespace

Testing

Manual testing

Tested login/logout on both iOS and Android emulators

Affected code checklist

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