Unlinking a user with user.unlink(fromProvider:) from Apple provider does not trigger IDTokenDidChangeListenerHandle

[shawnkoh]

[REQUIRED] Step 1: Describe your environment

• Xcode version: 12.5
• Firebase SDK version: 8.0.0
• Installation method: Swift Package Manager
• Firebase Component: Auth

[REQUIRED] Step 2: Describe the problem

Unlinking a user with user.unlink(fromProvider:) from Apple provider does not trigger IDTokenDidChangeListenerHandle.

Steps to reproduce:

0. Create a listener with addIDTokenDidChangeListener and store a reference to it. Add a print statement inside the listener's closure.
1. Create an anonymous user
2. Sign in with Apple
3. Calculate Credential from ASAuthorization
4. Call user.link(with: credential)
   IDTokenDidChangeListenerHandle will fire (as observed by the print statement)
5. Call user.unlink(fromProvider: "apple.com")
   IDTokenDidChangeListenerHandle will NOT fire (as observed by the lack of a print statement).

Related issues and pull requests:

#245
#249
#325

[weixifan]

Thank you for filing this. I've filed an internal issue b/188591834 to track.