Enable with CocoaPods `:configuration` (#28796)

Summary:
~~⚠️ Depends on https://github.com/facebook/flipper/pull/1086 and a new Flipper release.~~
Fixes https://github.com/facebook/react-native/commit/17f025bc26da13da795845a3f7daee65563420c0#commitcomment-38831234

Currently user’s are being told to add a definition of the `FB_SONARKIT_ENABLED` macro and examples, including those in stock React Native templates, set this for the user by making use of a `post_install` hook in the user’s `Podfile`. This leads to confusion, fragile code [when a user’s project dir structure deviates from vanilla], and is ultimately not necessary as CocoaPods already has dedicated mechanisms to:

* specify build settings (through the `xcconfig` property);
* and selectively include certain pods only in certain build configurations (e.g. debug).

## Changelog

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[iOS] [Changed] - Entirely control Flipper being enabled through inclusion in Podfile and optionally limiting to certain build configurations using the `:configuration` directive.
Pull Request resolved: https://github.com/facebook/react-native/pull/28796

Test Plan: Tested using the changes of https://github.com/facebook/flipper/pull/1086 in a new app that uses RN `master`.

Reviewed By: priteshrnandgaonkar

Differential Revision: D21449754

Pulled By: passy

fbshipit-source-id: 9ff7c7f4ffc32b364b1edd82b94e0b80c3997625