Introduce RCTInitializing

Summary:
## Motivation
NativeModules need a way to execute custom initialization code. The current way to achieve this is by overriding the setBridge: method. This doesn't work in bridgeless mode, because in bridgeless mode, the TurboModule system doesn't (and shouldn't) invoke the setBridge: method.

## Changes
This diff introduces a new protocol RCTInitializing that NativeModules can conform to. Once they conform to this protocol, they can implement the `- (void)initialize;` method. Both the NativeModule and TurboModule infra will execute this method towards the end of NativeModule setup.

## Questions
Why don't we just override the NativeModule init method?
1. **Backwards compatibility:** The legacy NativeModule infra requires that [modules that override their init method](https://www.internalfb.com/code/fbsource/[8098212b4e3733f0b469f11f3e61c5249a47f9f5]/xplat/js/react-native-github/React/Base/RCTModuleData.mm?lines=82-83) [must be initialized on the main thread](https://www.internalfb.com/code/fbsource/[8098212b4e3733f0b469f11f3e61c5249a47f9f5]/xplat/js/react-native-github/React/Base/RCTModuleData.mm?lines=85). We ported this over [to the TurboModule system](https://www.internalfb.com/code/fbsource/[d58a07e9ae90951d382224c28cb78c32c7cda80d]/xplat/js/react-native-github/ReactCommon/react/nativemodule/core/platform/ios/RCTTurboModuleManager.mm?lines=684%2C686). *We cannot require all NativeModules that need custom initialization to be initialized on the main thread.*
2. **More flexibility:** A custom `- (void)initialize` hook can be invoked by the NativeModule infra after the NativeModule is prepared (i.e: the bridge, module registry, view registry, etc. is attached). The same isn't true of an overriden init method, which has to be invoked when the object is created. This means that a custom `- (void)initialize` hook can use the bridge/method queue, etc., but an overriden init method cannot.

What about Android?
- After this diff, NativeModules can implement the `initialize` method to initialize themselves, and the `invalidate` method to invalidate themselves. This unifies initialization/invalidation across both iOS and Android, because on Android, TurboModules can also implement the same methods:

https://www.internalfb.com/code/fbsource/[d58a07e9ae90951d382224c28cb78c32c7cda80d]/xplat/js/react-native-github/ReactAndroid/src/main/java/com/facebook/react/turbomodule/core/interfaces/TurboModule.java?lines=11-17

Changelog: [iOS][Added] - Introduce RCTInitializing to allow NativeModules to initialize themselves

Reviewed By: fkgozali

Differential Revision: D28435078

fbshipit-source-id: 1a4a95e30f0a8b32b0844e9b9beee274afcbb577

React/Base/RCTInitializing.h

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <Foundation/Foundation.h>
+
+@protocol RCTInitializing <NSObject>
+
+- (void)initialize;
+
+@end