Summary:
The core React Native codebase already has full support for CocoaPods. However, `react-native link` doesn’t play nicely with CocoaPods, so installing third-party libs from the RN ecosystem is really hard.
This change will allow to link projects that contains its own `.podspec` file to CocoaPods-based projects. In case `link` detect `Podfile` in `iOS` directory, it will look for related `.podspec` file in linked project directory, and add it to `Podfile`. If `Podfile` and `.podspec` files are not present, it will fall back to previous implementation.
**Test Plan**
1. Build a React Native project where the iOS part uses CocoaPods to manage its dependencies. The most common scenario here is to have React Native be a Pod dependency, among others.
2. Install a RN-related library, that contains `.podspec` file, with `react-native link` (as an example it could be: [react-native-maps](https://github.com/airbnb/react-native-maps)
3. Building the resulting iOS workspace should succeed (and there should be new entry in `Podfile`)
Closes https://github.com/facebook/react-native/pull/15460
Differential Revision: D6078649
Pulled By: hramos
fbshipit-source-id: 9651085875892fd66299563ca0e42fb2bcc00825
Miron Pawlik