feat: add fabric-bridge node for RDMA/inter-host MXL flow sharing

- New fabric-bridge node: initiator mode reads local MXL flow and
  transfers via Fabrics (RDMA/TCP); target mode receives remote data
  and commits to local MXL flow writer
- Add set_mxl_instance/get_mxl_instance to Node base class
- CMake option DMF_BUILD_FABRICS=OFF by default (needs libfabric)
- Target publishes target_info in status for out-of-band exchange
- Initiator receives target_info via config parameter

libs/dmf-node/include/dmf-node/node.hpp

@@ -27,6 +27,12 @@ public:
     virtual void on_remove_reader(const std::string& port_id) = 0;
     virtual void process() = 0;
     virtual nlohmann::json status() const = 0;
+
+    void set_mxl_instance(mxlInstance instance) { mxl_instance_ = instance; }
+    mxlInstance get_mxl_instance() const { return mxl_instance_; }
+
+private:
+    mxlInstance mxl_instance_ = nullptr;
 };
 
 } // namespace dmf_node