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.js

@@ -56,6 +56,12 @@ export type ValueStruct = {|
 
 export type CustomHostObject = {};
 
+export type BinaryTreeNode = {
+  left?: BinaryTreeNode,
+  value: number,
+  right?: BinaryTreeNode,
+};
+
 export type GraphNode = {
   label: string,
   neighbors?: Array<GraphNode>,
@@ -68,6 +74,7 @@ export interface Spec extends TurboModule {
   +getCustomEnum: (arg: EnumInt) => EnumInt;
   +getCustomHostObject: () => CustomHostObject;
   +consumeCustomHostObject: (customHostObject: CustomHostObject) => string;
+  +getBinaryTreeNode: (arg: BinaryTreeNode) => BinaryTreeNode;
   +getGraphNode: (arg: GraphNode) => GraphNode;
   +getNumEnum: (arg: EnumInt) => EnumFloat;
   +getStrEnum: (arg: EnumNone) => EnumStr;