Added backwards compatible use of prebuild through cocoapods (#52252)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52252

Instead of declaring two different sets of Pods for prebuilt and build from source, this commit now keeps the pod structure the same for both modes so that consuming libraries can expect to have the same pods and header files available - without this, libraries would have to be updated to take advantage of the prebuilds.

This PR does:
- Added React-Core-prebuilt as a pod in React-Core if prebuilt is enabled
- Simplified react_native_pods to keep pods structure and add React-Core-prebuilt pod if prebuilts are enabled
- Added function for selecting source sets based on prebuilt/build from source

To be able to function both in prebuilt and in regular build from source mode, all podspecs are now using the switch function podspec_sources so that they only include header files if we are in prebuild mode.

Also added React-Core-prebuilt as dependency on React-Core if we are in prebuilt mode so that we install the React.XCFramework.

## Changelog:

[IOS] [FIXED] - Added backwards compatible use of prebuild through cocoapods

Pull Request resolved: https://github.com/facebook/react-native/pull/52223

Test Plan:
Tested in RN-Tester both with and without prebuild.

Rollback Plan:

Reviewed By: cortinico

Differential Revision: D77296047

Pulled By: cipolleschi

fbshipit-source-id: f3eb4d56b2a78bfc8e10ad852746be1ceaf828b2