Function transfer_download 

pub async fn transfer_download(
    profile_id: ProfileId,
    bucket: BucketId,
    key: String,
    dest_path: String,
    queue: State<'_, TransferQueueHandle>,
    pool: State<'_, S3ClientPoolHandle>,
    locks: State<'_, LockRegistryHandle>,
    store: State<'_, ProfileStoreHandle>,
    log: State<'_, NotificationLogHandle>,
    settings: State<'_, SettingsHandle>,
    channel: AppHandle,
) -> Result<String, AppError>

Initiate a streaming download of key from bucket to dest_path.

Returns the request_id (UUID v4) immediately.