[firebase_crashlytics] Implement successor of deprecated firebase_crashlytics setUserIdentifier() method.

[tijanirf]

I tested setUserIdentifier() method on my project and it didn't work. So I reported it a bug to firebase and they told me it was deprecated since 15 November 2020 (source)

I tested on firebase_crashlytics: ^0.2.4 but it's not there, I hope it got released soon. Thank you in advance.

P.S: I don't know which section to post this properly, I hope it's not in the wrong section.