Implement trace_offer

[acolytec3]

We should implement a portal_*TraceOffer endpoint so we can participate in some benchmarking that's been developed by @KolbyML.

Behavior should follow the existing portal_*Offer endpoint except return a trace object that looks like this (in Rust)

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub enum OfferTrace {
    /// Offer was successful, all accepted content keys in bitlist were transferred
    Success(BitList<typenum::U64>),
    /// Peer is not interested in any of the offered content keys
    Declined,
    /// This offer failed, perhaps locally or from a timeout or transfer failure
    Failed,
}