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/src/node_runner.cpp

@@ -71,6 +71,8 @@ int NodeRunner::exec(std::unique_ptr<Node> node) {
     }
     spdlog::info("MXL instance created on domain: {}", mxl_domain_);
 
+    node->set_mxl_instance(mxl_instance_);
+
     mxlGarbageCollectFlows(mxl_instance_);
 
     if (!config_str_.empty()) {