expand on the `DownloadProgressInitialState` struct

[ramfox]

The struct contains more fields than what we currently provide. Let's get this on par with iroh core.

[ramfox]

/// Accumulated progress state of a transfer.
#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq)]
pub struct TransferState {
    /// The root blob of this transfer (may be a hash seq),
    pub root: BlobState,
    /// Whether we are connected to a node
    pub connected: bool,
    /// Children if the root blob is a hash seq, empty for raw blobs
    pub children: HashMap<NonZeroU64, BlobState>,
    /// Child being transferred at the moment.
    pub current: Option<BlobId>,
    /// Progress ids for individual blobs.
    pub progress_id_to_blob: HashMap<ProgressId, BlobId>,
}