Add a BinaryTreeNode example for Cxx TMs (#41767)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41767

Changelog: [Internal]

Adds a simple example showing a direct recursive node in a Cxx TM.

Currently we can't auto-generate [the necessary C++ Types](https://reactnative.dev/docs/next/the-new-architecture/cxx-custom-types#struct-generator) - but we can add it later if this scenarios becomes really common.

Direct recursive nodes, can't be value types - it would require infinite memory. Hence they are nullable and managed by a smart pointer.

Reviewed By: rshest

Differential Revision: D51784136

fbshipit-source-id: f6f0710d03583bdf1e6e72ba42d8df7f8ff8d915

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.cpp

@@ -65,6 +65,12 @@ std::string NativeCxxModuleExample::consumeCustomHostObject(
   return value->a_ + std::to_string(value->b_);
 }
 
+BinaryTreeNode NativeCxxModuleExample::getBinaryTreeNode(
+    jsi::Runtime& rt,
+    BinaryTreeNode arg) {
+  return arg;
+}
+
 GraphNode NativeCxxModuleExample::getGraphNode(
     jsi::Runtime& rt,
     GraphNode arg) {