Add delegate/callback on crash report sent

### Feature proposal

* Firebase Component: Crashlytics

My feature request relates to [Crashlytics crash sending completion handler](https://github.com/firebase/firebase-ios-sdk/issues/5199)

We need to send application logs when app crashed. We can do it with non-fatal crashes, but we can't do it with regular crashes.

So my proposal is to extend related feature request, i.e. get some notification not only when `sendUnsentReports` were executed, but also when internal crashlog sending mechanism worked.