fix: prevent error when react extension was already created (#43694)

Summary:
Currently the react-native-gradle-plugin does not allow the "react" plugin extension to already exist when running its apply block. I had a use-case where I wanted to create a new gradle plugin which would take care of applying the react plugin including setting some of its options. Without the change in this PR, this would currently turn into a build failure.

## Changelog:

[ANDROID] [FIXED] - prevent error when the "react" extension was already created by another gradle plugin

Pull Request resolved: https://github.com/facebook/react-native/pull/43694

Reviewed By: rshest

Differential Revision: D55478611

Pulled By: zeyap

fbshipit-source-id: cc743a99cb72ed315d21c52597efd5ee92a3be62

packages/react-native-gradle-plugin/src/main/kotlin/com/facebook/react/ReactPlugin.kt

@@ -38,7 +38,10 @@ import org.gradle.internal.jvm.Jvm
 class ReactPlugin : Plugin<Project> {
   override fun apply(project: Project) {
     checkJvmVersion(project)
-    val extension = project.extensions.create("react", ReactExtension::class.java, project)
+    val extension =
+        project.rootProject.extensions.findByType(ReactExtension::class.java)
+            ?: project.rootProject.extensions.create("react", ReactExtension::class.java, project)
+
     checkIfNewArchFlagIsSet(project, extension)
 
     // We register a private extension on the rootProject so that project wide configs