[Rust] Don't leak the callbacks within DownloadInstance::perform_custom_request

[bdash]

I'm not entirely clear why this function copies the callbacks to the heap given that BNPerformCustomRequest is synchronous. Rather than changing that, I added an explicit Box::from_raw to ensure it is deallocated. This matches what is done within perform_request.

[emesare]

I think we probably should just avoid boxing it altogether and pass it by ref into the function: 966097b

[bdash]

@emesare is going to fix the leak via 966097b in the near future.