Add idTokenChanges to MockFirebaseAuth

[vinifig]

Hi, I changed the stream listener for my application authentication and the mock doesnt have a implementation for it

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following NoSuchMethodError was thrown running a test:
Class 'MockFirebaseAuth' has no instance method 'idTokenChanges' with matching arguments.
Receiver: Instance of 'MockFirebaseAuth'
Tried calling: idTokenChanges()
Found: idTokenChanges() => Stream<User?>

[atn832]

Fixed by @vinifig in #125 and released as version 0.15.1. https://pub.dev/packages/firebase_auth_mocks/changelog#0151. Thank you!