Now that the design doc is approved, we need to get working on implementation. We'll start with the protocol scope and then add the control plane/service discovery bits as they get approved.

• Double-HBONE client and (dummy) server implementation in echo #54295
• Add service hostname HBONE :authority support to ztunnel servers ztunnel#1422
• Add service hostname HBONE :authority support to waypoints servers #54306
• Add service hostname HBONE :authority support to sidecars servers
• Implement double-hbone client in ztunnel (we shouldn't need it in waypoints) ztunnel#1423
• Implement double-hbone server/forwarder in east-west gateway #54883