feat: Add getConcretePropsShared() (#48710)

Summary:
Adds `getConcretePropsShared()` to `ConcreteShadowNode.h`.

I need this in Nitro Views to update state in-place without throwing away the old state object and creating a new one each time.

From reading the code it seems like you use this pattern a lot tho where you create new State objects each time - so let me know if my thing is a bad idea..

## Changelog:

- [INTERNAL] [ADDED] Add `getConcretePropsShared()` to `ConcreteShadowNode.h`

<!-- Help reviewers and the release process by writing your own changelog entry.

Pick one each for the category and type tags:

[ANDROID|GENERAL|IOS|INTERNAL] [BREAKING|ADDED|CHANGED|DEPRECATED|REMOVED|FIXED|SECURITY] - Message

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests

Pull Request resolved: https://github.com/facebook/react-native/pull/48710

Test Plan: Use in Nitro

Reviewed By: sammy-SC

Differential Revision: D68495697

Pulled By: javache

fbshipit-source-id: e2caa34befcaef2191ec161442c40596cc7de132

packages/react-native/ReactCommon/react/renderer/core/ConcreteShadowNode.h

@@ -105,6 +105,16 @@ class ConcreteShadowNode : public BaseShadowNodeT {
     return static_cast<const ConcreteProps&>(*props_);
   }
 
+  /*
+   * Returns a concrete props object associated with the node, as a shared_ptr.
+   * Thread-safe after the node is sealed.
+   */
+  const std::shared_ptr<const ConcreteProps>& getConcreteSharedProps() const {
+    react_native_assert(
+        BaseShadowNodeT::props_ && "Props must not be `nullptr`.");
+    return std::static_pointer_cast<const ConcreteProps>(props_);
+  }
+
   /*
    * Returns a concrete event emitter object associated with the node.
    * Thread-safe after the node is sealed.