Cxx TurboModules > Add example to return a JS function from Cxx to JS (#41385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41385

Changelog: Internal

Adding a Cxx TM example which adds a listener and returns a subscription to remove that listener from the TM.

You should be able to use this with React Hooks - https://legacy.reactjs.org/docs/hooks-reference.html

E.g.

```
useEffect(() => {
  const subscription =  NativeCxxModuleExample.setValueCallbackWithSubscription(
          callbackValue => // use it
        );
  return subscription;
});
```

Reviewed By: shwanton

Differential Revision: D50473063

fbshipit-source-id: 4e9b92aeccff1771eb4ffad6bdaa20ba7f18435f

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.h

@@ -15,6 +15,7 @@
 #include <AppSpecs/AppSpecsJSI.h>
 #endif
 #include <memory>
+#include <optional>
 #include <set>
 #include <string>
 #include <vector>
@@ -107,6 +108,10 @@ class NativeCxxModuleExample
       jsi::Runtime& rt,
       AsyncCallback<std::string> callback);
 
+  std::function<void()> setValueCallbackWithSubscription(
+      jsi::Runtime& rt,
+      AsyncCallback<std::string> callback);
+
   std::vector<std::optional<ObjectStruct>> getArray(
       jsi::Runtime& rt,
       std::vector<std::optional<ObjectStruct>> arg);
@@ -169,6 +174,9 @@ class NativeCxxModuleExample
   ObjectStruct getObjectAssert(jsi::Runtime& rt, ObjectStruct arg);
 
   AsyncPromise<jsi::Value> promiseAssert(jsi::Runtime& rt);
+
+ private:
+  std::optional<AsyncCallback<std::string>> valueCallback_;
 };
 
 } // namespace facebook::react