Add hook that allows compression

[P1n3appl3]

We'd like to compress our minidumps. Currently to do so we have to flush and then re-read the minidump file from disk in on_minidump_created before compressing the whole thing and re-writing the file.

Ideally there'd be a way to add an adapter or make the server handler generic over something that implements Write. Then we could provide a zstd::stream::write::Encoder and do the compression inline.