After JS schedules new backup compaction creation we want to keep track of the promise_id so we can resolve/reject it after the operation. But the logic is split Rust -> C++ (compaction creation) -> Rust (upload to backup service) so we need to keep a map from backupID to the promise id.

Tested with the later diff but things that touch this code in particular:

• tested that the promise resolved after successful upload
• tested that the promise rejected with a correct message after an error on the Rust side
• tested that the promise rejected with a correct message after an error on the C++ side

The testing code looks roughly like this:

if let Err(err) = rust_op_that_can_fail().await {
  compaction_upload_promises::resolve(&backup_id, Err(err));
  return;
}

let (future_id, future) = future_manager::new_future::<()>().await;
cpp_that_can_fail(future_id);
if let Err(err) = future.await {
  compaction_upload_promises::resolve(&backup_id, Err(err));
  return;
}

compaction_upload_promises::resolve(&backup_id, Ok(()));