Add Send Failure Callback API

[naftaly]

Despite the newer features designed to limit report sizes, reports can still occasionally exceed the limit, resulting in them being silently deleted without being sent. When this happens frequently, there’s no way to detect it, which can lead to ticket prioritization problems and potential business losses.

This PR introduces a new callback, onSendFailureBlock, which is triggered when a report is dropped by the system without this being the intention of the sender.

I understand that Bugsnag typically doesn’t accept new features via PRs, but this one is straightforward and could provide significant value to end users. I’d greatly appreciate it if you could consider merging it as is or making any necessary adjustments before merging.

Thank you!