redistribution: refund_identity field (#636)

## 📝 Summary

Adds refund identity

## 💡 Motivation and Context

<!--- (Optional) Why is this change required? What problem does it
solve? Remove this section if not applicable. -->

---

## ✅ I have completed the following steps:

* [ ] Run `make lint`
* [ ] Run `make test`
* [ ] Added tests (if applicable)

Author: dvush

crates/rbuilder/src/backtest/build_block/synthetic_orders.rs

@@ -95,6 +95,7 @@ impl<ConfigType: LiveBuilderConfig> SyntheticOrdersSource<ConfigType> {
                 signer: None,
                 metadata: Metadata {
                     received_at_timestamp: time::OffsetDateTime::from_unix_timestamp(0).unwrap(),
+                    refund_identity: None,
                 },
                 dropping_tx_hashes: Default::default(),
                 refund: None,

crates/rbuilder/src/backtest/redistribute/mod.rs

@@ -1180,6 +1180,10 @@ fn order_redistribution_address(
     order: &Order,
     protect_signers: &[Address],
 ) -> Option<(Address, bool)> {
+    if let Some(refund_identity) = order.metadata().refund_identity {
+        return Some((refund_identity, false));
+    }
+
     let signer = match order.signer() {
         Some(signer) => signer,
         None => {

crates/rbuilder/src/primitives/mod.rs

@@ -36,12 +36,14 @@ use uuid::Uuid;
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Metadata {
     pub received_at_timestamp: time::OffsetDateTime,
+    pub refund_identity: Option<Address>,
 }
 
 impl Metadata {
     pub fn with_current_received_at() -> Self {
         Self {
             received_at_timestamp: time::OffsetDateTime::now_utc(),
+            refund_identity: None,
         }
     }
 }