FR: Crashlytics crash sending completion handler (or delegate messages)

### Feature proposal

* Firebase Component: Crashlytics

Sometimes our app is stuck on some state where it will crash upon startup. And therefore no crash reports are sent to Firebase. We want to add fake splash screen on startup and check if there are any unsent crash reports and send them. We want to continue app after crashes are sent.

So it would be great if Crashlytics could accept a completion handler for `sendUnsentReports` method

Start by locating the Crashlytics sendUnsentReports API and reading its current behavior and related delegate or callback handling. Done means callers can know when unsent reports have finished sending and safely continue startup afterward, including the crash-on-startup scenario described here.

Written by the indexing model from the issue text.