How should an in-cluster user-owned gateway identify an actor?

**Question:**
For egress traffic from an actor to an in-cluster user-owned gateway (not Substrate's egress gateway), how should the user-owned gateway identify the actor?

**Option 1:**
@haiyanmeng did a raw exploration in #1219 by modifying the Substrate egress gateway to forward the actor identity to cluster-local origins, without realizing that token exchange is included in the [egress policy](https://docs.google.com/document/d/1s2klDhbF2mB5sslwUyZBdyXD7JqRt8FWONwJ7wgsomE/edit?tab=t.0) now.

**Option 2:**
@keithmattix and @EItanya pointed out that the token exchange feature proposed in the [egress policy](https://docs.google.com/document/d/1s2klDhbF2mB5sslwUyZBdyXD7JqRt8FWONwJ7wgsomE/edit?tab=t.0) can be a good fit for this use case.

cc @keithmattix @EItanya @LiorLieberman @bowei